COD-39 attachment history drawer

Stacks on COD-38: accumulates a per-session attachment history and exposes it
through a slide-in drawer with an unread badge, so attachments stay reachable
after their cards are dismissed.

Backend:
- session-attachment-history: history state — dedupe by source path / relative
  path, newest-first, 100-item cap, and externalPath sanitization (the absolute
  host path is server-private and never leaves toState()).
- session.ts: _attachmentHistory + getter (sanitized) / upsert / restore /
  getAttachmentHistoryForPersist; restored from saved state in the constructor.
- file-routes: GET /attachments (list — resolves each entry to live metadata +
  routes; external entries are re-registered) and GET /attachments/:id
  (metadata poll). The by-id route guards via the registry's TOCTOU-safe
  resolveServableAttachmentPath.
- server.ts: detected/registered attachments upsert into history and persist;
  the private (externalPath-bearing) history rides on disk under
  __attachmentHistory, separate from the sanitized public copy, and is restored
  on mux-session recovery.
- types/session.ts: SessionAttachmentHistoryItem + SessionState.attachmentHistory.

Frontend:
- panels-ui: the drawer (lazy-built), unread badge, list render with per-item
  preview/download/open/"Card" (reshow) actions, and live refresh of the open
  drawer on new detections.
- app.js: history state + per-session badge/cleanup wiring.
- index.html / styles.css / mobile.css: header button + badge and the drawer.

Verified: tsc / eslint / prettier / frontend-syntax / public-assets clean; new
history-module unit tests pass; full test:ci green (2866 passed); badge, drawer
open/render/reshow/close verified in-browser.

src/web/public/mobile.css

@@ -2346,3 +2346,47 @@ html.mobile-init .file-browser-panel {
     display: block;
   }
 }
+
+@media (max-width: 430px) {
+  /* Attachment history (COD-18): full-screen sheet on phones */
+  .attachment-history-drawer {
+    top: 0;
+    bottom: 0;
+    width: 100%;
+    max-width: 100%;
+    height: 100vh;
+    height: 100dvh;
+    border-left: none;
+    border-radius: 0;
+    padding-top: var(--safe-area-top);
+    padding-left: var(--safe-area-left);
+    padding-right: var(--safe-area-right);
+    padding-bottom: var(--safe-area-bottom);
+  }
+
+  .attachment-history-header {
+    padding: 12px;
+  }
+
+  .attachment-history-list {
+    padding: 6px;
+  }
+
+  .attachment-history-item {
+    grid-template-columns: 104px minmax(0, 1fr);
+    gap: 8px;
+    padding: 8px 6px;
+  }
+
+  .attachment-history-thumb {
+    width: 104px;
+  }
+
+  .attachment-history-actions {
+    gap: 4px;
+  }
+
+  .attachment-history-actions button {
+    padding: 4px 7px;
+  }
+}