fix(cases): custom-folder create landing fixes (#535)

- Route test hygiene: each test works in its own mkdtemp folder, every
  deletion goes through safeRmHomeTree, and the suite refuses to start
  outside test/setup.ts's temp HOME, so a raw `npx vitest` can no longer
  delete a real ~/projects or the live linked-cases registry.
- Path policy: the symlink-resolved target is also judged against the
  resolved home, data dir and system roots (home reached through a link,
  macOS /etc -> /private/etc); test expectations are realpath-safe.
- Refuse a target equal to or inside the caller's or the shared cases
  directory, pointing at plain Create New (it would list twice, and
  deleting the local copy removes files).
- The registry re-read comment no longer claims to prevent the
  lost-update race; documented as narrowing it, like /api/cases/link.
- UI: the success toast names the folder the server created, the
  "under ~/codeman-cases" blurb and name hint change while a custom
  folder is ticked, a "/" parent previews and sends /<name> instead of
  an empty path, and the new labels have zh-CN entries.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/index.html

@@ -3023,11 +3023,11 @@
               <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="3" y="3" width="18" height="18" rx="2"/><path d="M12 8v8M8 12h8"/></svg>
               <h2>Create New</h2>
             </div>
-            <p class="set-section-blurb">A fresh workspace under ~/codeman-cases, scaffolded with its own CLAUDE.md.</p>
+            <p class="set-section-blurb" id="newCaseBlurb">A fresh workspace under ~/codeman-cases, scaffolded with its own CLAUDE.md.</p>
             <div class="form-row">
               <label>Case Name</label>
               <input type="text" id="newCaseName" placeholder="my-project" pattern="[a-zA-Z0-9_-]+" autocomplete="off" autocapitalize="off" spellcheck="false" oninput="app.updateNewCasePathPreview()">
-              <span class="form-hint">Letters, numbers, hyphens, underscores only. Created in ~/codeman-cases/</span>
+              <span class="form-hint" id="newCaseNameHint">Letters, numbers, hyphens, underscores only. Created in ~/codeman-cases/</span>
             </div>
             <div class="form-row">
               <label>Description (optional)</label>