Org assignment history, mobile support, and a correctness pass

Data model
- employee_assignments records org placement over time (valid_from/valid_to),
  written by a trigger on `employees` rather than inside each RPC: ~70
  `update employees` statements spread over fifteen migrations mean per-call
  bookkeeping would miss paths today and again with every future RPC. A
  partial unique index enforces the one-open-interval invariant the trigger
  relies on when closing the current row.
- The Organigramm gains a Stichtag (default today). Membership comes from
  entry/exit/karenz, past placement from the new history, future placement
  projected from pending_org_changes. Placements predating the migration are
  backfilled with today's values and flagged as such in the UI, since
  employee_history only ever stored free text and cannot be reconstructed.

Correctness
- Reports and exports silently truncated at PostgREST's 1000-row cap
  (db.max_rows); employee_history is already past it at ~800 staff. Every
  whole-table read now pages explicitly.
- XLSX date cells were a day early: ExcelJS converts a Date to an Excel
  serial straight off getTime(), so a Date built at local midnight lands on
  the previous day's serial in any positive-offset zone.
- Date handling is pinned to Europe/Vienna throughout, and date-only strings
  are formatted without a Date round-trip. The dashboard's YTD window was
  built by round-tripping a local Date through toISOString(), which shifted
  it a day early and dropped 31 December entirely.
- Export routes parsed measure/group/split/eventType with unchecked `as`
  casts, so an unknown value reached column headers as `undefined` and the
  Content-Disposition filename. Parsed against the label maps now, with the
  filename slugged as a backstop.
- toXlsx keyed columns by header text, silently dropping the second of any
  two columns sharing a name — split columns take their header from data.
- The org chart tree walks had no cycle guard; nothing in the schema forbids
  a manager_id cycle, and one would hang the tab rather than misreport.
- The login page reflected ?error= verbatim, letting anyone put arbitrary
  text on the real sign-in screen; messages are looked up by code now.
- React Flow needs elementsSelectable on, or it sets pointer-events:none on
  the whole node and the expand control stops responding.

UI
- Mobile: the shell was unusable below lg — a fixed 236px margin pushed
  content off-screen with no mobile navigation at all. The sidebar is now a
  drawer, dvh replaces vh, safe-area insets are honoured, inputs are 16px so
  iOS stops zooming on focus, and form grids stack.
- Org chart nodes redesigned: per-kind accent stripes and icons, vacant
  roles called out, expand control moved to the bottom edge carrying the
  child count.
- Pagination is windowed; it previously rendered one link per page (54 for
  the employee list, unbounded for the audit log).
- Positions page reduced to open positions with a single "Besetzen" action.
- The employee Organisation tab links into the org chart focused on that
  person, reusing the chart's existing search-match highlighting.

Also included, uncommitted until now
- Dependants, HR notes, academic titles, split address fields, position
  validity and role/employment fields, with their migrations and UI.
- Docker/compose deployment setup, data-model and security-review docs.

components/orgchart/graphLayout.ts

@@ -0,0 +1,60 @@
+import dagre from "@dagrejs/dagre";
+import type { ChartNode, ChartNodeKind } from "./types";
+
+// Must match what OrgChartNode actually renders: Dagre reserves exactly this
+// much space per node, so a box that grows past it overlaps its neighbour.
+export const NODE_DIMENSIONS: Record<ChartNodeKind, { width: number; height: number }> = {
+  person: { width: 268, height: 80 },
+  role: { width: 268, height: 80 },
+  group: { width: 244, height: 62 },
+  vacancy: { width: 244, height: 62 },
+};
+
+export type VisibleEdge = { id: string; source: string; target: string };
+
+// Collapsing a node means excluding its descendants here, not CSS-hiding a
+// layout computed for the full tree — Dagre only ever lays out what's
+// actually visible, which is what keeps this usable at ~800 employees.
+export function collectVisible(tree: ChartNode[], isExpanded: (id: string) => boolean): { visibleNodes: ChartNode[]; visibleEdges: VisibleEdge[] } {
+  const visibleNodes: ChartNode[] = [];
+  const visibleEdges: VisibleEdge[] = [];
+
+  function walk(n: ChartNode) {
+    visibleNodes.push(n);
+    if (n.children.length > 0 && isExpanded(n.id)) {
+      for (const c of n.children) {
+        visibleEdges.push({ id: `${n.id}->${c.id}`, source: n.id, target: c.id });
+        walk(c);
+      }
+    }
+  }
+  for (const root of tree) walk(root);
+
+  return { visibleNodes, visibleEdges };
+}
+
+// Dagre returns center points; React Flow positions nodes by their top-left
+// corner, hence the width/height/2 offset below.
+export function layoutWithDagre(visibleNodes: ChartNode[], visibleEdges: VisibleEdge[]): Map<string, { x: number; y: number }> {
+  const g = new dagre.graphlib.Graph();
+  // ranksep leaves room for the expand button that overhangs each node's
+  // bottom edge; nodesep keeps sibling cards from visually merging.
+  g.setGraph({ rankdir: "TB", nodesep: 40, ranksep: 88 });
+  g.setDefaultEdgeLabel(() => ({}));
+
+  for (const n of visibleNodes) {
+    const { width, height } = NODE_DIMENSIONS[n.kind];
+    g.setNode(n.id, { width, height });
+  }
+  for (const e of visibleEdges) g.setEdge(e.source, e.target);
+
+  dagre.layout(g);
+
+  const positions = new Map<string, { x: number; y: number }>();
+  for (const n of visibleNodes) {
+    const { width, height } = NODE_DIMENSIONS[n.kind];
+    const { x, y } = g.node(n.id);
+    positions.set(n.id, { x: x - width / 2, y: y - height / 2 });
+  }
+  return positions;
+}