Add user-defined invoice set structure dialog (ADR 0016)

Lets a back-office user correct/define which invoice line belongs to
which "set" and how deep it is nested, instead of relying only on the
implicit "everything after a Set header until the next Set" grouping
fds__prepInvoice derives. Sets may contain sets; every change applies
immediately to the cached draft session and is reflected in the editor
and PDF (ADR 0006/0008).

Backend:
- InvoiceSetStructure: pure parent-link forest (assign, change level,
  create, place/move-sibling, depth-first linearise + level recompute).
- InvoiceDraftSetStructureDeltas: new inv/dpatch targets set.assign,
  set.level, set.create, set.move.
- InvoiceSetPricing.Build is now order-preserving and nesting-aware
  (governed by the nearest converted ancestor); the set-item switch
  sums a header's transitive descendants so nested conversions never
  double-count.
- FuchsPdf indents nested set members per level, mirroring the editor.

Frontend:
- New fis.inv_sets.js: "Set-Struktur bearbeiten" dialog — per-block
  table of all lines with drag & drop reorder (before/after/into),
  ▲▼ order and ◀▶ level buttons, fold/unfold per set (view-only), and
  a create bar for new Set/Text lines. Posts one delta per action; the
  dialog is a pure view of the server state (ADR 0006).
- _Layout.cshtml / fis_main.js: cache-bust the dynamically imported
  per-module bundles (web/fis.<mdl>.<locale>.js/.css) with an asset
  version derived from the newest bundle write time, so a gulp rebuild
  is picked up without a stale cached module.

Docs: ADR 0016, live-draft-editing concept, INVOICE_SET_PRICING.md,
USER_GUIDE.md, ARCHITECTURE.md updated. Tests: InvoiceSetStructureTests
(new) + additions to InvoiceDraftServiceTests/InvoiceSetPricingTests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Fuchs/Docs/INVOICE_SET_PRICING.md

@@ -1,8 +1,10 @@
 # Invoice "Set" Pricing — Design & Front-/Back-end Contract
 
 > Governed by ADR [0009](Decisions/0009-block-setprice-modes-are-irreversible-mutations.md)
-> (the two menu modes) and ADR [0008](Decisions/0008-invoices-and-reminders-fully-backend-authoritative.md)
-> (everything server-side, PDF == editor). ADR 0009 **redefined** the two menu modes from the
+> (the two menu modes), ADR [0008](Decisions/0008-invoices-and-reminders-fully-backend-authoritative.md)
+> (everything server-side, PDF == editor) and ADR
+> [0016](Decisions/0016-user-defined-set-structure.md) (set membership is an explicit,
+> user-editable parent link; sets may nest — see "Set structure" below). ADR 0009 **redefined** the two menu modes from the
 > reversible, non-mutating display toggles this document previously described into irreversible,
 > per-service-request-block mutations — the text below reflects the redefinition.
 
@@ -40,6 +42,37 @@ for the full breakdown.
 > or removes (mode 3), and the set-item switch (mode 1) writes exactly the members' sum onto the
 > header.
 
+## Set structure (ADR 0016): which line belongs to which set
+
+`SetItmId` is no longer only what `fds__prepInvoice` derived from row order — it is the
+**explicit parent link** of every cached line, and the user can edit it:
+
+- **Initial structure** = the implicit derivation (every item after a zero-priced Set header
+  until the next one). Nothing changes for drafts the user never touches.
+- **"Set-Struktur bearbeiten"** (editor menu → `$inv.sets`, `fis.inv_sets.js`) shows every
+  line per service-request block (Pos., Typ, Menge, Bezeichnung, indented by level). Drag a
+  line onto a set → `set.assign { parent }` (it becomes the set's last member); drag it onto
+  the block heading / the "oberste Ebene" zone → `set.assign { parent: "" }` (pulled out, placed
+  after the subtree it came from). Drag a line to the **top/bottom edge** of another row →
+  `set.move { anchor, where: before|after }` (reorder: it becomes that row's sibling; a set
+  travels with its members, folded or not); ▲ / ▼ post `set.move` −1 / +1 (one step among
+  siblings). The ◀ / ▶ buttons post `set.level` −1 / +1 (outdent to the
+  grandparent / indent under the nearest preceding sibling set). Sets can be folded via the
+  triangle (view state only). The per-block bar creates a
+  new **Set** (zero-priced header, convertible via function 1) or **Text** line inside any set
+  or at the top level → `set.create { typ, title, parent }`.
+- **Sets may contain sets.** A set dropped onto another set keeps its own members (one level
+  deeper). Function 1 (`item.setprice`) sums the header's **transitive** descendants and nulls
+  them all; converting inner-then-outer or outer directly conserves the total either way.
+- **Only sets are drop targets; a line never leaves its block.** Cycles (a set onto its own
+  descendant) and no-op moves are rejected server-side (`InvoiceSetStructure`), the browser
+  merely greys them out.
+- **The server linearises** after each structural delta (depth-first: set, then its members,
+  recursively; siblings keep their order) and publishes `lvl`/`level`; `InvoiceSetPricing.Build`
+  is order-preserving, so editor and PDF show the same sequence and the same per-level indent.
+  Every change is applied immediately to the cached session and pushed back via
+  `draftReady`/`dstate`; the dialog re-renders from that state.
+
 ## Back-end
 
 > **Migration status (ADR 0009).** The code below still reflects the previous
@@ -51,9 +84,14 @@ for the full breakdown.
 > migration lands so it stays a faithful description of the code.
 
 - `Fuchs/code/InvoiceSetPricing.cs` — the authoritative transformation:
-  `SetDisplayMode` + `Build(items, mode)` → ordered `InvoiceSetLine`s, each with
-  `ShowPrice` and `IsSetHeader`. `ModeFromInvoiceOptions(...)` reads the mode
+  `SetDisplayMode` + `Build(items, mode)` → `InvoiceSetLine`s **in input order**, each with
+  `ShowPrice`, `IsSetHeader` and `Level` (nesting depth). A line is governed by its nearest
+  *converted* ancestor set (blank in SetPrice, dropped in SetOnly); an unconverted header is
+  blank with individually priced members. `ModeFromInvoiceOptions(...)` reads the mode
   from the invoice options. Fully covered by `Fuchs.Tests/InvoiceSetPricingTests.cs`.
+- `Fuchs/code/InvoiceSetStructure.cs` — the pure set forest (parent links, levels, depth-first
+  linearisation, assign / change level / create); `Fuchs/Services/InvoiceDraftSetStructureDeltas.cs`
+  maps the `set.*` patch targets onto it. Covered by `InvoiceSetStructureTests` + `InvoiceDraftServiceTests`.
 - `FuchsPdf.ApplyInvoice` renders through `InvoiceSetPricing.Build`: lines with
   `ShowPrice == false` render **blank** price/total cells (not `0,00 €`), and the
   set header line is rendered emphasised. Invoices without sets pass through
@@ -82,11 +120,12 @@ Wired in `Fuchs/js/intranet/modules/fis.inv_shared.js` (bundled to
    C# PDF. This change closes that gap for **all** invoices, not just sets.)
 
 3. **Set flags (function 1 only)** — `invSumUpdate` tags items as it builds `items[]`:
-   an item with `type === 'set'` is a header (`id` = its set id); a member item's
-   `setId` is taken directly from the server-computed `SetItmId` field on the row
-   (`rrx.SetItmId`, populated by `fds__prepInvoice`'s `[SetItmID]` window function,
-   anchored on the still-unconverted, zero-priced `'set'` header that owns it) —
-   **not** re-derived from row order in the browser. `mfr__items` itself still has no
+   an item with `type === 'set'` is a header (`id` = its set id); a member's
+   `setId` is taken directly from the `SetItmId` field on the row
+   (`rrx.SetItmId` — initially populated by `fds__prepInvoice`'s `[SetItmID]` window function,
+   anchored on the still-unconverted, zero-priced `'set'` header that owns it, and thereafter
+   maintained by the server per ADR 0016, including for a **nested set**, which is a member of
+   its parent set) — **not** re-derived from row order in the browser. `mfr__items` itself still has no
    explicit member link; `fds__prepInvoice` computes `SetItmId` per request from the
    item list, so only items the server actually attributes to a set are tagged, and
    unrelated items following a set in the list are never swept in. The header row's