fix(attachments): address review findings on attachment history drawer (#121)

Follow-up fixes applied during review of PR #121 (all confirmed minor/nit;
no blockers). Security posture verified sound (externalPath never leaves
toState()/the list route; re-registration runs the guard).

- fix(recovery): restoreAttachmentHistory now skips malformed/legacy saved
  items (null, non-object, missing source/fileName) instead of throwing inside
  the Session constructor — a corrupt __attachmentHistory entry could otherwise
  abort the entire mux-recovery loop. (P1)
- fix(routes): the attachment-list route degrades a single failing entry to
  {missing:true} instead of failing the whole drawer. (INT-4)
- fix(ui): give the attachments header button a positioning context so the
  unread badge anchors to the icon, not the header bar. (F1/CSS-1)
- fix(ui): cancel the debounced history refresh on drawer close and guard it
  against a stale session/closed drawer. (F3)
- fix(ui): re-show ("Card") of a detected item now uses the item's own
  timestamp so the cardId is stable — focuses the existing card instead of
  stacking duplicates. (F4)
- fix(ui): Escape now closes the drawer, matching every other panel. (UX-1)
- fix(ui): badge shows "99+" past 99 (was an inconsistent 100/99 cap). (BADGE-1)
- style: drop the duplicate @keyframes notif-badge-pulse (dead CSS). (INT-1/CSS-3)
- style: empty-state used three undefined CSS custom properties
  (--text-primary/--border-color/--bg-tertiary) → use the defined
  --text/--border-light/--bg-input tokens. (CSS-2)
- test: add constructor restore round-trip + malformed-item resilience tests.

Deferred (noted for author): broadcasting the full 100-item history in every
session-state SSE event (payload bloat), "unread" badge semantics, making the
header button opt-in, and app.inject route tests for the two new endpoints.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/session.ts

@@ -919,6 +919,11 @@ export class Session extends EventEmitter {
   restoreAttachmentHistory(history: SessionAttachmentHistoryItem[] | undefined): void {
     this._attachmentHistory = [];
     for (const item of [...(history ?? [])].reverse()) {
+      // Guard against malformed/legacy on-disk entries (null, non-object, or
+      // missing required fields). historyKey() dereferences source/fileName, so
+      // a bad item would otherwise throw inside the constructor and abort the
+      // entire mux-recovery loop.
+      if (!item || typeof item !== 'object' || !item.source || !item.fileName) continue;
       this.upsertAttachmentHistory(item);
     }
   }