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.

lib/export.ts

@@ -1,4 +1,5 @@
 import ExcelJS from "exceljs";
+import { todayIso } from "./format";
 
 // Shared by every /api/export/* route: define columns once as { header, get },
 // get both a semicolon CSV (Excel-DE friendly) and a real .xlsx workbook from
@@ -38,8 +39,12 @@ export function toCsv<T>(rows: T[], columns: ExportColumn<T>[]): string {
   return "" + lines.join("\r\n");
 }
 
+// Anchored at UTC midnight, not local: ExcelJS converts a JS Date to an Excel
+// serial straight off getTime() with no timezone adjustment, so a Date built
+// at *local* midnight in a positive-offset zone (Vienna) lands on the previous
+// day's serial and every date cell in the workbook renders one day early.
 function parseIsoDate(value: string): Date | null {
-  const d = new Date(`${value}T00:00:00`);
+  const d = new Date(`${value}T00:00:00Z`);
   return Number.isNaN(d.getTime()) ? null : d;
 }
 
@@ -47,9 +52,12 @@ export async function toXlsx<T>(rows: T[], columns: ExportColumn<T>[], sheetName
   const workbook = new ExcelJS.Workbook();
   const sheet = workbook.addWorksheet(sheetName.slice(0, 31));
 
-  sheet.columns = columns.map((c) => ({
+  // Keyed by position, not by header text: split columns take their header
+  // from the data (a team name, a weekday), so two columns can legitimately
+  // collide — and ExcelJS silently drops the second one when two share a key.
+  sheet.columns = columns.map((c, i) => ({
     header: c.header,
-    key: c.header,
+    key: String(i),
     width: Math.min(40, Math.max(12, c.header.length + 4)),
     style: c.kind === "date" ? { numFmt: "dd.mm.yyyy" } : undefined,
   }));
@@ -59,9 +67,9 @@ export async function toXlsx<T>(rows: T[], columns: ExportColumn<T>[], sheetName
 
   for (const row of rows) {
     const record: Record<string, string | number | boolean | Date | null> = {};
-    for (const c of columns) {
+    for (const [i, c] of columns.entries()) {
       const value = c.get(row);
-      record[c.header] =
+      record[String(i)] =
         c.kind === "date" && typeof value === "string" && value
           ? (parseIsoDate(value) ?? value)
           : typeof value === "string"
@@ -75,9 +83,18 @@ export async function toXlsx<T>(rows: T[], columns: ExportColumn<T>[], sheetName
   return new Uint8Array(written);
 }
 
+// The base carries values that originate in the query string (event type,
+// measure, dimension) and ends up inside a Content-Disposition header, so it
+// is reduced to a filename-safe slug here rather than trusted. Callers also
+// validate those params; this is the backstop that makes header injection
+// impossible regardless.
 export function exportFilename(base: string, format: "csv" | "xlsx"): string {
-  const today = new Date().toISOString().slice(0, 10);
-  return `${base}-${today}.${format}`;
+  const slug = base
+    .normalize("NFKD")
+    .replace(/[^a-zA-Z0-9._-]+/g, "-")
+    .replace(/^-+|-+$/g, "")
+    .slice(0, 80);
+  return `${slug || "export"}-${todayIso()}.${format}`;
 }
 
 export function exportResponseHeaders(filename: string, format: "csv" | "xlsx"): HeadersInit {