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/positions/CreatePositionModal.tsx

@@ -6,6 +6,7 @@ import { createPosition, searchSuperiors, type SuperiorSearchResult } from "@/ac
 import { Lookup } from "@/components/ui/Lookup";
 import { Modal } from "@/components/ui/Modal";
 import { useToast } from "@/components/ui/Toast";
+import { todayIso } from "@/lib/format";
 import type { Database } from "@/lib/supabase/types";
 
 type Team = Database["public"]["Tables"]["teams"]["Row"];
@@ -17,6 +18,7 @@ export function CreatePositionModal({ open, onClose, teams }: { open: boolean; o
   const [isLead, setIsLead] = useState(false);
   const [superior, setSuperior] = useState<SuperiorSearchResult | null>(null);
   const [teamId, setTeamId] = useState("");
+  const [validFrom, setValidFrom] = useState(todayIso);
   const [pending, setPending] = useState(false);
 
   function reset() {
@@ -24,10 +26,11 @@ export function CreatePositionModal({ open, onClose, teams }: { open: boolean; o
     setIsLead(false);
     setSuperior(null);
     setTeamId("");
+    setValidFrom(todayIso());
   }
 
   async function handleSubmit() {
-    if (!title || !superior || (isLead && !teamId)) {
+    if (!title || !superior || !validFrom || (isLead && !teamId)) {
       showToast("Bitte alle Pflichtfelder ausfüllen.", "error");
       return;
     }
@@ -37,6 +40,7 @@ export function CreatePositionModal({ open, onClose, teams }: { open: boolean; o
       superior_employee_id: superior.id,
       is_lead: isLead,
       team_id: isLead ? teamId : undefined,
+      valid_from: validFrom,
     });
     setPending(false);
     if (result.success) {
@@ -130,6 +134,15 @@ export function CreatePositionModal({ open, onClose, teams }: { open: boolean; o
             </select>
           </div>
         )}
+        <div>
+          <label className="mb-1 block text-sm font-semibold text-ink">Gültig ab*</label>
+          <input
+            type="date"
+            value={validFrom}
+            onChange={(e) => setValidFrom(e.target.value)}
+            className="w-full rounded border border-border px-3 py-2 text-sm"
+          />
+        </div>
       </div>
     </Modal>
   );