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-attachment-history.ts

@@ -0,0 +1,92 @@
+import { createHash } from 'node:crypto';
+import { basename, extname } from 'node:path';
+import type { SessionAttachmentHistoryItem } from './types/session.js';
+import type { AttachmentDetectedEvent } from './types/tools.js';
+import { getAttachmentType } from './attachment-registry.js';
+
+export const ATTACHMENT_HISTORY_LIMIT = 100;
+
+export interface ExternalAttachmentHistoryInput {
+  sessionId: string;
+  externalPath: string;
+  fileName?: string;
+  extension?: string;
+  size: number;
+  mtimeMs?: number;
+  timestamp?: number;
+}
+
+export function normalizeAttachmentExtension(extensionOrPath: string): string {
+  const value = extensionOrPath.startsWith('.') ? extensionOrPath : extname(extensionOrPath) || extensionOrPath;
+  return value.toLowerCase().replace(/^\./, '');
+}
+
+function historyKey(item: SessionAttachmentHistoryItem): string {
+  if (item.source === 'external' && item.externalPath) {
+    return `external:${item.externalPath}`;
+  }
+  return `detected:${item.relativePath || item.fileName}`;
+}
+
+function safeExternalHistoryId(item: SessionAttachmentHistoryItem): string {
+  const source = item.externalPath || item.id || item.fileName;
+  const digest = createHash('sha256').update(source).digest('hex').slice(0, 16);
+  return `external:${digest}:${item.fileName}`;
+}
+
+export function sanitizeAttachmentHistoryItem(item: SessionAttachmentHistoryItem): SessionAttachmentHistoryItem {
+  const { externalPath: _externalPath, ...safe } = item;
+  return {
+    ...safe,
+    id: item.source === 'external' ? safeExternalHistoryId(item) : item.id,
+  };
+}
+
+export function sanitizeAttachmentHistory(
+  history: readonly SessionAttachmentHistoryItem[]
+): SessionAttachmentHistoryItem[] {
+  return history.map(sanitizeAttachmentHistoryItem);
+}
+
+export function upsertAttachmentHistory(
+  history: readonly SessionAttachmentHistoryItem[],
+  item: SessionAttachmentHistoryItem
+): SessionAttachmentHistoryItem[] {
+  const nextKey = historyKey(item);
+  return [item, ...history.filter((existing) => historyKey(existing) !== nextKey)].slice(0, ATTACHMENT_HISTORY_LIMIT);
+}
+
+export function buildDetectedAttachmentHistoryItem(event: AttachmentDetectedEvent): SessionAttachmentHistoryItem {
+  return {
+    id: `detected:${event.relativePath || event.fileName}`,
+    sessionId: event.sessionId,
+    fileName: event.fileName,
+    extension: normalizeAttachmentExtension(event.extension),
+    attachmentType: event.attachmentType,
+    size: event.size,
+    mtimeMs: 0,
+    timestamp: event.timestamp,
+    source: 'detected',
+    relativePath: event.relativePath,
+  };
+}
+
+export function buildExternalAttachmentHistoryItem(
+  input: ExternalAttachmentHistoryInput
+): SessionAttachmentHistoryItem {
+  const extension = normalizeAttachmentExtension(input.extension || input.fileName || input.externalPath);
+  return {
+    id: `external:${createHash('sha256').update(input.externalPath).digest('hex').slice(0, 16)}:${
+      input.fileName || basename(input.externalPath)
+    }`,
+    sessionId: input.sessionId,
+    fileName: input.fileName || basename(input.externalPath),
+    extension,
+    attachmentType: getAttachmentType(extension),
+    size: input.size,
+    mtimeMs: input.mtimeMs ?? 0,
+    timestamp: input.timestamp ?? Date.now(),
+    source: 'external',
+    externalPath: input.externalPath,
+  };
+}