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/types/session.ts

@@ -25,6 +25,7 @@
  */
 
 import type { RespawnConfig } from './respawn.js';
+import type { AttachmentDetectedType } from './tools.js';
 
 /** Status of a Claude session */
 export type SessionStatus = 'idle' | 'busy' | 'stopped' | 'error';
@@ -101,6 +102,40 @@ export interface SessionConfig {
  */
 export type SessionColor = 'default' | 'red' | 'orange' | 'yellow' | 'green' | 'blue' | 'purple' | 'pink';
 
+export type SessionAttachmentHistorySource = 'detected' | 'external';
+
+/**
+ * Session-scoped attachment history entry.
+ *
+ * `externalPath` is server-private. It may be present in the internal persisted
+ * history copy, but API-bound session state must sanitize it before returning
+ * to the browser.
+ */
+export interface SessionAttachmentHistoryItem {
+  /** Stable history identity used for dedupe and list rendering */
+  id: string;
+  /** Codeman session ID this item belongs to */
+  sessionId: string;
+  /** Display filename */
+  fileName: string;
+  /** Lowercase extension without a leading dot */
+  extension: string;
+  /** Viewer category used by the web UI */
+  attachmentType: AttachmentDetectedType;
+  /** File size in bytes */
+  size: number;
+  /** Last modified timestamp in milliseconds, if known */
+  mtimeMs: number;
+  /** Last time this attachment was seen or explicitly published */
+  timestamp: number;
+  /** How the attachment entered the session */
+  source: SessionAttachmentHistorySource;
+  /** Workspace-relative path for detected session files */
+  relativePath?: string;
+  /** Server-private absolute path for explicitly published external files */
+  externalPath?: string;
+}
+
 /**
  * Current state of a session
  */
@@ -183,6 +218,8 @@ export interface SessionState {
   resumeSessionId?: string;
   /** Claude CLI effort level (soft default via --settings, switchable in-session via /effort) */
   effort?: EffortLevel;
+  /** Sanitized per-session attachment history. */
+  attachmentHistory?: SessionAttachmentHistoryItem[];
 }
 
 /**