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/AsOfPicker.tsx

@@ -0,0 +1,76 @@
+"use client";
+
+import { CalendarClock } from "lucide-react";
+import { usePathname, useRouter, useSearchParams } from "next/navigation";
+import { fmtDate } from "@/lib/format";
+
+type AsOfPickerProps = {
+  asOf: string;
+  today: string;
+  /** Placements projected from effective-dated changes not yet applied. */
+  projectedCount: number;
+  /** Earliest date the assignment history covers; before that it is today's placement. */
+  historyStartsAt: string | null;
+};
+
+export function AsOfPicker({ asOf, today, projectedCount, historyStartsAt }: AsOfPickerProps) {
+  const router = useRouter();
+  const pathname = usePathname();
+  const searchParams = useSearchParams();
+
+  function setAsOf(value: string | undefined) {
+    const sp = new URLSearchParams(searchParams.toString());
+    if (value && value !== today) sp.set("asOf", value);
+    else sp.delete("asOf");
+    router.push(sp.size > 0 ? `${pathname}?${sp}` : pathname, { scroll: false });
+  }
+
+  const isToday = asOf === today;
+  const isFuture = asOf > today;
+  // Assignments only started being recorded when the history table was
+  // introduced; asking for a date before that yields today's placement for
+  // everyone, which is worth saying out loud rather than quietly implying.
+  const beforeHistory = historyStartsAt !== null && asOf < historyStartsAt;
+
+  return (
+    <div className="rounded border border-border bg-white p-3">
+      <div className="flex flex-wrap items-center gap-x-3 gap-y-2">
+        <label htmlFor="orgchart-asof" className="flex items-center gap-1.5 text-sm font-semibold text-ink">
+          <CalendarClock className="h-4 w-4 text-ink-muted" />
+          Stichtag
+        </label>
+        <input
+          id="orgchart-asof"
+          type="date"
+          value={asOf}
+          onChange={(e) => setAsOf(e.target.value || undefined)}
+          className="rounded border border-border px-3 py-2 text-sm"
+        />
+        {!isToday && (
+          <button type="button" onClick={() => setAsOf(undefined)} className="text-xs font-semibold text-brand-700 hover:underline">
+            Heute
+          </button>
+        )}
+        <span className="text-xs text-ink-muted">
+          {isToday ? "Aktuelle Organisationsstruktur." : `Struktur zum ${fmtDate(asOf)}.`}
+        </span>
+      </div>
+
+      {isFuture && (
+        <p className="mt-2 rounded bg-info-bg px-3 py-2 text-xs text-info-text">
+          Vorschau: {projectedCount > 0
+            ? `${projectedCount} geplante Versetzung(en)/Reorganisation(en) sind eingerechnet.`
+            : "Für diesen Zeitraum sind keine Versetzungen vorgemerkt."}{" "}
+          Ein-/Austritte und Karenzen sind berücksichtigt.
+        </p>
+      )}
+
+      {beforeHistory && (
+        <p className="mt-2 rounded bg-warning-bg px-3 py-2 text-xs text-warning-text">
+          Vor dem {fmtDate(historyStartsAt)} wurde die Zuordnungshistorie noch nicht aufgezeichnet. Wer beschäftigt war,
+          stimmt; Team und Vorgesetzte zeigen für diesen Stichtag die heutige Zuordnung.
+        </p>
+      )}
+    </div>
+  );
+}