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/modules/fis.inv_shared.scss

@@ -81,6 +81,37 @@
 			}
 		}
 		
+		.emailccfrm {
+			position: absolute;
+			font-size: 0.7rem;
+			top: 2.2rem;
+			left: 0.5rem;
+			width: auto;
+			height: auto;
+			cursor: pointer;
+
+			.content {
+				color: #000;
+			}
+
+			/* Unlike the other optional fields (plain "-" placeholder), this one names the action —
+			   there is no default "Zusatzempfänger" from MFR, so an empty state must read as an
+			   offer to add one, not as missing data. */
+			.content:empty::before {
+				content: 'Zusatzempfänger hinzufügen';
+				min-width: 5rem;
+				height: 1rem;
+				display: block;
+				position: absolute;
+			}
+
+			&:hover {
+				div.axf {
+					display: block;
+				}
+			}
+		}
+
 		.admfrm {
 			position: absolute;
 			right: 3rem;
@@ -250,6 +281,12 @@
 				&.sethdr td.txt {
 					font-weight: bold;
 				}
+				/* Nested set members (ADR 0016): indent per level, mirroring the PDF's per-level indent. */
+				@for $i from 1 through 6 {
+					&.lvl#{$i} td.txt {
+						padding-left: calc(0.4rem + #{$i * 1.4rem});
+					}
+				}
 			}
 
 			> tfoot > tr {
@@ -493,3 +530,71 @@
 		color: #445;
 	}
 }
+
+/* ── Set structure dialog (ADR 0016, see $inv.sets in fis.inv_sets.js) ─────────────────── */
+.modal .setstruct {
+	&.busy { opacity: 0.6; pointer-events: none; }
+
+	div.shint { font-size: 0.85rem; color: #666; margin: 0 0 0.8rem 0; }
+
+	div.sblock {
+		margin: 0 0 1.4rem 0;
+
+		div.sbhd {
+			font-weight: bold;
+			padding: 0.4rem 0.6rem;
+			border-top: 1.5px solid #000;
+			border-bottom: 1px solid #ddd;
+			&.dropok { background-color: #e6f2e6; outline: 2px dashed #5a5; }
+		}
+
+		table.stbl {
+			width: 100%;
+			border-collapse: collapse;
+
+			th { text-align: left; font-size: 0.8rem; color: #666; padding: 0.3rem 0.4rem; border-bottom: 1px solid #ddd; }
+			td { padding: 0.3rem 0.4rem; border-bottom: 1px solid #f0f0f0; vertical-align: middle; }
+			td.grip { width: 1.6rem; cursor: grab; color: #999; }
+			td.pos, td.qty { width: 4rem; white-space: nowrap; }
+			td.typ { width: 7rem; white-space: nowrap; color: #666; font-size: 0.85rem;
+				span.fold { cursor: pointer; margin-right: 0.35rem; color: #555; display: inline-block; width: 1rem; }
+			}
+			td.ttl {
+				span.ind { display: inline-block; }
+				span.seti { margin-right: 0.3rem; color: #888; }
+				span.cnt { color: #999; font-weight: normal; font-size: 0.85rem; }
+			}
+			td.act { width: 7.5rem; white-space: nowrap; text-align: right;
+				div.ibtn { display: inline-block; cursor: pointer; padding: 0 0.25rem; &.disabled { opacity: 0.25; cursor: default; } }
+			}
+
+			tr.srow {
+				&.isset td.ttl { font-weight: bold; }
+				&.hid { display: none; }
+				&.folded td.ttl { font-style: italic; }
+				&.dragging { opacity: 0.4; }
+				&.dropok td { background-color: #e6f2e6; }
+				&.dropbefore td { border-top: 3px solid #5a5; }
+				&.dropafter td { border-bottom: 3px solid #5a5; }
+			}
+		}
+
+		div.sroot {
+			margin: 0.3rem 0 0.5rem 0;
+			padding: 0.4rem 0.6rem;
+			border: 1px dashed #bbb;
+			color: #888;
+			font-size: 0.85rem;
+			&.dropok { background-color: #e6f2e6; border-color: #5a5; color: #363; }
+		}
+
+		div.screate {
+			display: flex; flex-wrap: wrap; align-items: center; gap: 0.4rem;
+			padding: 0.4rem 0.6rem;
+			background-color: #f7f7f7;
+			span.lbl { font-size: 0.85rem; color: #666; }
+			select, input[type="text"] { padding: 0.2rem 0.3rem; }
+			input[type="text"] { flex: 1 1 14rem; min-width: 10rem; }
+		}
+	}
+}