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/Views/Shared/_Layout.cshtml

@@ -15,6 +15,18 @@
     }
     authinfo["requestedaccount"] = "";
     authinfo["accountrequired"]  = false;
+    // Cache-buster for the per-module bundles ($ocms.getScript loads web/fis.<mdl>.<locale>.js and
+    // web/fis.<mdl>.css dynamically, so asp-append-version cannot cover them): the newest write
+    // time of any module bundle, appended as ?v=… by fis_main.js, so a gulp rebuild is picked up
+    // on the next page load instead of a stale cached module.
+    try
+    {
+        var webDir = new System.IO.DirectoryInfo(System.IO.Path.Combine(HostEnvironment.WebRootPath, "web"));
+        authinfo["assetversion"] = webDir.Exists
+            ? webDir.GetFiles("fis.*").Select(f => f.LastWriteTimeUtc.Ticks).DefaultIfEmpty(0L).Max().ToString()
+            : "0";
+    }
+    catch { authinfo["assetversion"] = "0"; }
 
     string appName  = ViewData["AppName"]  as string ?? "Fuchs Intranet";
     string fullName = ViewData["FullName"] as string ?? "";