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/ui/Modal.tsx

@@ -25,21 +25,27 @@ export function Modal({ open, onClose, title, children, footer, widthClassName =
   if (!open) return null;
 
   return (
-    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/40 p-4">
-      <div className={`flex max-h-[85vh] w-full flex-col rounded bg-white shadow-xl ${widthClassName}`}>
-        <div className="flex items-center justify-between border-b border-border px-6 py-4">
+    // Bottom-aligned on a phone (thumb reach, and it clears the keyboard when
+    // a field is focused), centred from sm up.
+    <div className="fixed inset-0 z-50 flex items-end justify-center bg-black/40 p-0 sm:items-center sm:p-4">
+      <div
+        role="dialog"
+        aria-modal="true"
+        aria-label={title}
+        className={`flex max-h-[92dvh] w-full flex-col rounded-t-xl bg-white shadow-xl sm:max-h-[85dvh] sm:rounded ${widthClassName}`}
+      >
+        <div className="flex shrink-0 items-center justify-between border-b border-border px-4 py-3 sm:px-6 sm:py-4">
           <h2 className="text-lg font-bold text-ink">{title}</h2>
-          <button
-            type="button"
-            onClick={onClose}
-            aria-label="Schließen"
-            className="rounded p-1 text-ink-muted hover:bg-surface"
-          >
+          <button type="button" onClick={onClose} aria-label="Schließen" className="-mr-1 rounded p-2 text-ink-muted hover:bg-surface">
             <X className="h-5 w-5" />
           </button>
         </div>
-        <div className="flex-1 overflow-y-auto px-6 py-4">{children}</div>
-        {footer && <div className="flex items-center justify-end gap-2 border-t border-border px-6 py-4">{footer}</div>}
+        <div className="flex-1 overflow-y-auto overscroll-contain px-4 py-4 sm:px-6">{children}</div>
+        {footer && (
+          <div className="flex shrink-0 flex-wrap items-center justify-end gap-2 border-t border-border px-4 py-3 pb-[max(0.75rem,env(safe-area-inset-bottom))] sm:px-6 sm:py-4">
+            {footer}
+          </div>
+        )}
       </div>
     </div>
   );