COD-38 document attachment previews + thumbnails (attachment cards)

Builds on the COD-37 registry: surfaces detected/registered attachments as
dismissible cards with a first-page thumbnail and an inline preview — the
consumer the registry PR deliberately deferred.

Backend:
- document-thumbnailer: first-page PNG thumbnails (PNG passthrough; PDF via
  pdftoppm; Office via the preview cache).
- document-preview-cache: disk-cached DOCX/PPTX -> PDF conversion (LibreOffice
  / PowerShell COM), in-flight dedup, multi-converter fallback.
- file-routes: serveConvertedPreview / serveThumbnail + four routes —
  GET .../attachments/:id/preview, .../thumbnail and the workspace-path
  file-preview / file-thumbnail. Reuses the registry's TOCTOU-safe
  resolveServableAttachmentPath, so previews stream the freshly-resolved path.
- server: enrich detected attachment events with a thumbnail route.
- image-watcher: .png now routes to attachment:detected — this PR adds the card
  consumer, so the screenshot popup is no longer its only handler.

Frontend:
- panels-ui: attachment cards (addAttachmentCard, lazy stack, Clear-all,
  per-session cleanup) plus a 3-arg openFilePreview that renders registered
  attachments inline (image/PDF) or via the server-converted PDF (docx/pptx).
- app.js: wire attachment:detected -> _onAttachmentDetected and card state.
- styles: attachment-card + stack styling.

Verified: tsc / eslint / prettier / frontend-syntax clean; new thumbnailer +
preview-cache unit tests pass; full test:ci green (2861 passed); card render +
preview overlay + dismiss verified in-browser.

test/document-thumbnailer.test.ts

@@ -0,0 +1,96 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { execFile } from 'node:child_process';
+import fs from 'node:fs/promises';
+import { generateFirstPageThumbnail } from '../src/document-thumbnailer.js';
+import { clearDocumentPreviewCache } from '../src/document-preview-cache.js';
+
+vi.mock('node:child_process', () => ({
+  execFile: vi.fn((_cmd, _args, _options, callback) => {
+    callback(null, { stdout: '', stderr: '' });
+    return {};
+  }),
+}));
+
+vi.mock('node:fs/promises', () => ({
+  default: {
+    stat: vi.fn(async () => ({ size: 100, isFile: () => true })),
+    readFile: vi.fn(async () => Buffer.from('png')),
+    readdir: vi.fn(async () => ['converted.pdf']),
+    mkdir: vi.fn(async () => undefined),
+    mkdtemp: vi.fn(async () => '/tmp/codeman-thumb-test'),
+    rename: vi.fn(async () => undefined),
+    rm: vi.fn(async () => undefined),
+  },
+}));
+
+const mockedExecFile = vi.mocked(execFile);
+const mockedStat = vi.mocked(fs.stat);
+const mockedReadFile = vi.mocked(fs.readFile);
+const mockedReaddir = vi.mocked(fs.readdir);
+const mockedMkdir = vi.mocked(fs.mkdir);
+const mockedMkdtemp = vi.mocked(fs.mkdtemp);
+const mockedRename = vi.mocked(fs.rename);
+const mockedRm = vi.mocked(fs.rm);
+
+describe('document-thumbnailer', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    clearDocumentPreviewCache();
+    mockedStat.mockImplementation(async (path) => {
+      const pathText = String(path);
+      if (pathText.includes('codeman-document-preview-cache') && pathText.endsWith('.pdf')) {
+        throw new Error('ENOENT');
+      }
+
+      return { size: 500 * 1024 * 1024, mtimeMs: 12345, isFile: () => true } as never;
+    });
+    mockedReadFile.mockResolvedValue(Buffer.from('large thumbnail') as never);
+    mockedReaddir.mockResolvedValue(['converted.pdf'] as never);
+    mockedMkdir.mockResolvedValue(undefined as never);
+    mockedMkdtemp.mockResolvedValue('/tmp/codeman-thumb-test' as never);
+    mockedRename.mockResolvedValue(undefined as never);
+    mockedRm.mockResolvedValue(undefined as never);
+    mockedExecFile.mockImplementation((_cmd, _args, _options, callback) => {
+      callback(null, { stdout: '', stderr: '' });
+      return {} as never;
+    });
+  });
+
+  it('renders first-page thumbnails for large documents without an app-level size cap', async () => {
+    const result = await generateFirstPageThumbnail('/tmp/large-deck.pdf', 'pdf');
+
+    expect(result).toEqual({
+      content: Buffer.from('large thumbnail'),
+      contentType: 'image/png',
+    });
+    expect(mockedExecFile).toHaveBeenCalledWith(
+      'pdftoppm',
+      expect.arrayContaining(['-png', '-singlefile', '-f', '1', '-l', '1']),
+      expect.any(Object),
+      expect.any(Function)
+    );
+  });
+
+  it('renders Office thumbnails from the cached converted PDF after conversion cleanup', async () => {
+    mockedMkdtemp.mockImplementation(async (prefix) =>
+      String(prefix).includes('codeman-document-preview-cache')
+        ? '/tmp/codeman-document-preview-cache/work-test'
+        : '/tmp/codeman-thumb-pdf-test'
+    );
+    mockedExecFile.mockImplementation((_cmd, _args, _options, callback) => {
+      callback(null, { stdout: '', stderr: '' });
+      return {} as never;
+    });
+
+    const result = await generateFirstPageThumbnail('/tmp/deck.pptx', 'pptx');
+
+    expect(result).toEqual({
+      content: Buffer.from('large thumbnail'),
+      contentType: 'image/png',
+    });
+    const pdftoppmCall = mockedExecFile.mock.calls.find(([cmd]) => cmd === 'pdftoppm');
+    expect(pdftoppmCall?.[1]).toEqual(
+      expect.arrayContaining([expect.stringContaining('codeman-document-preview-cache')])
+    );
+  });
+});