feat(ui): opt-in entrance animations for tabs, terminal pane, agent windows and connection lines

All OFF by default (the `legacy` theme), so an untouched install behaves exactly
as before and every mark/apply hook short-circuits on its first line. Opt in via
App Settings > Appearance > Entrance Animations; per-surface control and a live
preview lab at ?animlab=1.

Surfaces and styles:
- Tabs: slide, pop, crt, unroll, boot, flip. A batch launched together cascades
  by a configurable stagger.
- Terminal pane: crt, boot, wipe, slide, fade.
- Agent windows: fly (the pre-existing tab-to-window flight, still the default),
  crt, materialize, unfold, beam, pop.
- Connection lines: draw, packet, fade.

Three constraints drove the design:

1. Tabs and connection lines are DESTROYED mid-animation on every re-render:
   _fullRenderSessionTabs() replaces the strip's innerHTML and
   _updateConnectionLinesImmediate() does `svg.innerHTML = ''`, both of which run
   constantly while sessions and agents spawn. Each is tracked by id and
   re-applied to the fresh element with a NEGATIVE animation-delay so it resumes
   at the same offset instead of restarting or snapping. Verified on the real
   path: a forced rebuild mid-draw resumed at -0.243s.

2. Terminal-pane styles animate transform/opacity/clip-path ONLY. xterm's
   FitAddon derives rows+cols from getComputedStyle(parent).width/height, the
   untransformed layout box, so transforms are invisible to it; animating
   width/height/padding would have resized the PTY. Verified by forcing
   fitAddon.fit() eight times mid-animation: dimensions held at 178x38.

3. A window entrance that transforms also moves the rect its connection line
   aims at (crt drifts it 109px, pop 81px). `beam` animates opacity/filter only
   (0px drift) so its line can draw toward a stable target; the others refresh
   the lines on animationend.

Also fixes: an agent window spawning hidden (its agent belongs to a background
tab) is display:none, so its animation never runs and animationend never fires,
which left the entrance class and its inline custom property stuck on the window
permanently. Hidden windows now skip the entrance entirely.

Styles persist to their own codeman:*Anim localStorage keys, keeping them
per-device without touching the .strict() SettingsUpdateSchema.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/index.html

@@ -1276,6 +1276,20 @@
                   </optgroup>
                 </select>
               </div>
+              <div class="settings-item settings-item-multiline" title="How new session tabs, the terminal pane, agent windows and their connection lines appear. This device only. For per-surface control and a live preview, add ?animlab=1 to the URL.">
+                <div class="settings-item-text">
+                  <span class="settings-item-label">Entrance Animations</span>
+                  <span class="settings-item-desc">How new tabs, panes and agent windows arrive</span>
+                </div>
+                <select id="appSettingsEntranceAnim" class="form-select settings-inline-select">
+                  <option value="legacy">Off (default)</option>
+                  <option value="terminal">Terminal (CRT)</option>
+                  <option value="beamdown">Beam down</option>
+                  <option value="quiet">Quiet</option>
+                  <option value="playful">Playful</option>
+                  <option value="custom">Custom (set in the lab)</option>
+                </select>
+              </div>
               <div class="settings-item" id="appSettingsWebglRendererItem" title="Use the GPU-accelerated WebGL terminal renderer (desktop only). Turn off to force the DOM renderer if you hit GPU glitches. Codeman also auto-falls-back to the DOM renderer after repeated GPU stalls.">
                 <span class="settings-item-label">WebGL Renderer</span>
                 <label class="switch switch-sm">
@@ -2626,6 +2640,7 @@
   <script defer src="admin-ui.js"></script>
   <script defer src="session-ui.js"></script>
   <script defer src="webview-tabs.js"></script>
+  <script defer src="entrance-animations.js"></script>
   <script defer src="ralph-wizard.js"></script>
   <script defer src="api-client.js"></script>
   <script defer src="subagent-windows.js"></script>