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/ARCHITECTURE.md

@@ -177,6 +177,8 @@ disable/failure). See [`Concepts/erechnung-output.md`](Concepts/erechnung-output
 Stateless services (`IPdfService`, `IBankingService`, `IMfrClientFactory`) are singletons; DB/request-scoped services are scoped (see `Program.cs`).
 The **Admin** module (`Do_Process_Admin`, `ISystemStatusService`) surfaces a live system-status/diagnostics page (host, SQL/Key Vault/blob/MFR connectivity, email config, test-email) restricted to `fds_sys` > 4 — see ADR [0011](Decisions/0011-admin-module-system-status.md) and the [concept doc](Concepts/admin-system-status.md).
 `FdsInvoiceData` / `FdsReminderData` are now **pure data holders** (parse + properties); loading, persistence and PDF generation live in the services (fully async — no `Task.Run(...).Wait()`).
+
+Invoice/reminder drafts are edited **backend-authoritatively**: the truth is an in-memory `InvoiceDraftSession` mutated by single deltas (`IInvoiceDraftService`), including the user-defined **set structure** (`InvoiceSetStructure` — parent links, nesting, levels; ADR [0016](Decisions/0016-user-defined-set-structure.md)) and the set-price operations (`InvoiceSetPricing`, ADR [0009](Decisions/0009-block-setprice-modes-are-irreversible-mutations.md)). See the [live draft editing concept](Concepts/live-draft-editing.md) and ADRs [0006](Decisions/0006-backend-authoritative-draft-editing.md)/[0008](Decisions/0008-invoices-and-reminders-fully-backend-authoritative.md).
 `FuchsPdf` / `FuchsVisualization` remain as static rendering libraries used *by* the services. The earlier static, controller-coupled helpers (`FuchsWidgets`, `FuchsReports`, `Banking`, `FuchsFdsEmail`) have been removed.
 
 ### 4.4 SQL-First Data Access