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/js/intranet/fis_main.js

@@ -17,8 +17,11 @@
                 }
                 $ocms[mdl].auth = auth;
                 if (auth.manage > 0) {
+                    /* ?v=<assetversion> (newest bundle write time, emitted by _Layout) busts the browser
+                       cache of the dynamically imported module bundle + css after a gulp rebuild. */
+                    let av = '?v=' + encodeURIComponent($ocms.auth.assetversion || '0');
                     $ocms.getScript({
-                        module: mdl, script: ['/web/fis', mdl, $ocms.auth.locale || 'de', 'js'].join('.'), css: ['/web/fis', mdl, 'css'].join('.'), condition: typeof $ocms[mdl].init2 !== 'function'
+                        module: mdl, script: ['/web/fis', mdl, $ocms.auth.locale || 'de', 'js'].join('.') + av, css: ['/web/fis', mdl, 'css'].join('.') + av, condition: typeof $ocms[mdl].init2 !== 'function'
                     }, function () { $ocms[mdl].init2(); });
                 }
             }, error: function () {