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/employees/tabs/VertragTab.tsx

@@ -13,14 +13,23 @@ const PAYGRADE_LABELS: Record<string, string> = {
 };
 
 export function VertragTab({ employee }: { employee: EmployeeRow }) {
+  const flags = [
+    employee.is_betriebsrat && "Betriebsrat",
+    employee.has_dienstwagen && "Dienstwagen",
+    employee.is_laterale_fuehrung && "Laterale Führung",
+    employee.is_c_level && "C-Level",
+  ].filter(Boolean);
   const rows: [string, string][] = [
     ["Eintrittsdatum", fmtDate(employee.entry_date)],
     ["Vertragsart", employee.contract_type === "befristet" ? `befristet bis ${fmtDate(employee.contract_end_date)}` : "unbefristet"],
-    ["Kollektivvertrag", "Metalltechnische Industrie"],
+    ["Kollektivvertrag", employee.collective_agreement ?? "–"],
     ["Beschäftigungsausmaß", employee.employment_type],
     ["Wochenstunden", `${employee.weekly_hours} h`],
     ["Urlaubsanspruch", "25 Tage"],
     ["Paygrade", PAYGRADE_LABELS[employee.paygrade] ?? employee.paygrade],
+    ["Angestellte:r / Arbeiter:in", employee.worker_type ?? "–"],
+    ["Arbeitstage", employee.work_days?.join(", ") || "–"],
+    ["Merkmale", flags.length > 0 ? flags.join(", ") : "–"],
   ];
   if (employee.exit_date) rows.push(["Austrittsdatum", fmtDate(employee.exit_date)]);