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.

app/(auth)/login/page.tsx

@@ -1,11 +1,24 @@
 import { login, logout } from "@/actions/auth";
 
+// The query string is attacker-controlled, so the login page renders a message
+// looked up by code rather than whatever text ?error= carries. Reflecting the
+// raw parameter let anyone put arbitrary wording ("Ihr Konto wurde gesperrt,
+// rufen Sie …") on the real, correctly-branded sign-in screen.
+const ERROR_MESSAGES = {
+  no_hr_access: "Kein HR-Zugriff. Bitte wenden Sie sich an eine:n bestehende:n HR-Benutzer:in.",
+  invalid_credentials: "E-Mail oder Passwort ist falsch.",
+} as const;
+
+type ErrorCode = keyof typeof ERROR_MESSAGES;
+
 type LoginPageProps = {
   searchParams: Promise<{ error?: string }>;
 };
 
 export default async function LoginPage({ searchParams }: LoginPageProps) {
-  const { error } = await searchParams;
+  const params = await searchParams;
+  const code = params.error && Object.hasOwn(ERROR_MESSAGES, params.error) ? (params.error as ErrorCode) : null;
+  const error = code ? ERROR_MESSAGES[code] : null;
 
   return (
     <div className="flex min-h-screen items-center justify-center bg-surface px-4">
@@ -16,11 +29,13 @@ export default async function LoginPage({ searchParams }: LoginPageProps) {
         {error && (
           <div role="alert" className="mt-4 rounded bg-danger-bg px-3 py-2 text-sm text-danger-text">
             {error}
-            <form action={logout} className="mt-2">
-              <button type="submit" className="text-xs font-semibold underline hover:no-underline">
-                Abmelden und mit anderem Konto versuchen
-              </button>
-            </form>
+            {code === "no_hr_access" && (
+              <form action={logout} className="mt-2">
+                <button type="submit" className="text-xs font-semibold underline hover:no-underline">
+                  Abmelden und mit anderem Konto versuchen
+                </button>
+              </form>
+            )}
           </div>
         )}