feat(docker): one-click "Run in Docker" case creation + Export button

- New POST /api/cases/docker-quickcreate: creates a normal case (folder in
  CASES_DIR, scaffolded CLAUDE.md + hooks) AND links it to a hardened container
  with default settings, auto-provisioning a shared `default` docker host — the
  user never touches host/image/network fields.
- Create New tab gains a "Run in isolated Docker container" checkbox; on submit it
  calls docker-quickcreate then auto-starts a claude session inside the container.
- Case Manage list gains an Export (full-image) button per docker case.
- SSE listeners for docker:exportComplete/exportFailed toast + refresh the exports
  list.

Verified end-to-end on the live instance: one-click create put the case in
~/codeman-cases/<name>, auto-created the default host, launched claude in the
container; export button produces a bundle; checkbox + button render (Playwright).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/public/index.html

@@ -1864,6 +1864,10 @@
               <label>Description (optional)</label>
               <input type="text" id="newCaseDescription" placeholder="A brief description..." autocomplete="off">
             </div>
+            <div class="form-row">
+              <label class="checkbox-row"><input type="checkbox" id="newCaseDocker"> 🐳 Run in an isolated Docker container</label>
+              <span class="form-hint">One click: creates the case folder AND a hardened container with default settings, then starts the session inside it. Requires the base image (<code>node scripts/build-agent-image.mjs</code>).</span>
+            </div>
           </div>
           <!-- Link Existing Tab -->
           <div class="modal-tab-content hidden" id="case-link">