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/session.ts

@@ -81,6 +81,11 @@ import { SessionAutoOps } from './session-auto-ops.js';
 import { detectUsageLimitPause } from './usage-limit-patterns.js';
 import { SessionTaskCache } from './session-task-cache.js';
 import { parseAttachmentMagicLinks } from './attachment-magic.js';
+import {
+  sanitizeAttachmentHistory,
+  upsertAttachmentHistory as upsertAttachmentHistoryList,
+} from './session-attachment-history.js';
+import type { SessionAttachmentHistoryItem } from './types/session.js';
 
 export type { BackgroundTask } from './task-tracker.js';
 export type { RalphTrackerState, RalphTodoItem, ActiveBashTool } from './types.js';
@@ -314,6 +319,7 @@ export class Session extends EventEmitter {
 
   // Bounded dedup set for terminal attachment magic-links already requested.
   private _attachmentMagicSeen = new Set<string>();
+  private _attachmentHistory: SessionAttachmentHistoryItem[] = [];
 
   // Nice prioritying configuration
   private _niceConfig: NiceConfig = { ...DEFAULT_NICE_CONFIG };
@@ -405,6 +411,8 @@ export class Session extends EventEmitter {
       envOverrides?: Record<string, string>;
       /** Claude CLI effort level (soft default via --settings, switchable in-session via /effort) */
       effort?: EffortLevel;
+      /** Restored per-session attachment history. May include server-private external paths. */
+      attachmentHistory?: SessionAttachmentHistoryItem[];
     }
   ) {
     super();
@@ -471,6 +479,9 @@ export class Session extends EventEmitter {
     if (config.effort && isEffortLevel(config.effort)) {
       this._effort = config.effort;
     }
+    if (config.attachmentHistory && config.attachmentHistory.length > 0) {
+      this.restoreAttachmentHistory(config.attachmentHistory);
+    }
 
     // Initialize task tracker and forward events (store handlers for cleanup)
     this._taskTracker = new TaskTracker();
@@ -897,6 +908,25 @@ export class Session extends EventEmitter {
     return this._status === 'idle' || this._status === 'busy';
   }
 
+  get attachmentHistory(): SessionAttachmentHistoryItem[] {
+    return sanitizeAttachmentHistory(this._attachmentHistory);
+  }
+
+  upsertAttachmentHistory(item: SessionAttachmentHistoryItem): void {
+    this._attachmentHistory = upsertAttachmentHistoryList(this._attachmentHistory, item);
+  }
+
+  restoreAttachmentHistory(history: SessionAttachmentHistoryItem[] | undefined): void {
+    this._attachmentHistory = [];
+    for (const item of [...(history ?? [])].reverse()) {
+      this.upsertAttachmentHistory(item);
+    }
+  }
+
+  getAttachmentHistoryForPersist(): SessionAttachmentHistoryItem[] | undefined {
+    return this._attachmentHistory.length > 0 ? this._attachmentHistory.map((item) => ({ ...item })) : undefined;
+  }
+
   toState(): SessionState {
     return {
       id: this.id,
@@ -936,6 +966,7 @@ export class Session extends EventEmitter {
       codexConfig: this._codexConfig,
       resumeSessionId: this._resumeSessionId,
       effort: this._effort,
+      attachmentHistory: this.attachmentHistory.length > 0 ? this.attachmentHistory : undefined,
       // envOverrides intentionally NOT on the public SessionState type — they must not
       // leak into SSE / GET /api/sessions broadcasts (schema allows OPENCODE_*, which
       // can carry secrets). For disk persistence, session-manager calls