[grade=B urn:ump:dbxmyhvr7cpkeu3mbfdevrco7utsok743udjsj7atih6qk4frdiq] fix(DC-168): calendar widget CSRF writes, error path, modal UX (audit batch 2)

Audit 2026-10-27 fixes (46-finding sweep, AUDIT-2026-10-27.md):
- P0-2: all 6 calendar API calls via secureFetch — raw fetch sent no
  X-CSRF-Token so every mutation (save/toggle/delete/test) was rejected
  403 [DC-100]; GETs gain the standard 15s timeout
- P0-6: refresh() catch called nonexistent errorHandler.log() — the
  error path itself threw; now logError(context, error)
- P0-7: modal sizing off inline min-width:560px onto .cal-modal-content
  with 768px override (inline style beat the mobile media query)
- refresh coalescing via _pendingForce (forced refreshes were dropped
  mid-flight); single fetch sequence on modal open; toggle/delete get
  resp.ok + try/catch + toast + disabled-during-flight
- modal on class-based .show: click-outside, Escape, focus move/return
  on all three dismissal paths (judge polish applied)
- chip honors Enter/Space (role=button); poll skips hidden tabs
- --cal-chip-when/--cal-when/--cal-dot re-scoped to :root (were scoped
  to elements that are never ancestors of .cal-dot — DC-168 WCAG dot
  colors never rendered)
- calendar-cache eventKey no longer throws RangeError on invalid start

Bundles rebuilt (sw dashcaddy-shell-7fc13e7762). status suite 68/68.
Judge B (polish items applied pre-commit): urn:ump:dbxmyhvr7cpkeu3mbfdevrco7utsok743udjsj7atih6qk4frdiq

status/js/calendar-cache.js

@@ -5,7 +5,12 @@
 'use strict';
 (function () {
   function eventKey(ev) {
-    const startIso = new Date(ev.start).toISOString();
+    // Audit 2026-10-27: a null/invalid start threw RangeError out of merge()
+    // and flipped the whole widget offline — degrade to a string key instead.
+    const d = new Date(ev.start);
+    const startIso = isNaN(d.getTime())
+      ? `invalid:${ev.start == null ? 'null' : String(ev.start)}`
+      : d.toISOString();
     return ev.uid ? `${ev.uid}@${startIso}` : `${startIso}|${ev.title || ''}`;
   }