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/app.js

@@ -800,6 +800,10 @@ class CodemanApp {
     this.applyLocalization();
     this.applyTabWrapSettings();
     this.applyMonitorVisibility();
+    // Must run before the first session:created can arrive: markSessionTabEntering()
+    // ignores ids until this sets up its state, which is what keeps the tabs
+    // restored on page load from animating.
+    this.initEntranceAnimations?.();
     // Remove mobile-init class now that JS has applied visibility settings.
     // The inline <script> in <head> added this to prevent flash-of-content on mobile.
     document.documentElement.classList.remove('mobile-init');
@@ -1563,6 +1567,12 @@ class CodemanApp {
       this.sessionOrder.push(data.id);
       this.saveSessionOrder();
     }
+    // Idempotent per id: the POST response and the session:created event both
+    // land here, and a batch launched together cascades in creation order.
+    this.markSessionTabEntering?.(data.id);
+    // The pane is one shared element, so it is only marked here and played when
+    // this session is actually selected (see selectSession).
+    this.markTerminalEntering?.(data.id);
     this.renderSessionTabs();
     this.updateCost();
     // Start stats polling when first session appears
@@ -3443,6 +3453,9 @@ class CodemanApp {
     }
 
     this.updateTabOverflowMode();
+    // After the wrap measurement: the `unroll` style starts tabs at max-width 0,
+    // so measuring mid-animation would decide the wrap on collapsed widths.
+    this._applyTabEntrances?.();
   }
 
   // Auto-wrap desktop session tabs to a second row when they overflow one row,
@@ -3569,6 +3582,9 @@ class CodemanApp {
     // toggle (applyTabWrapSettings calls this) which would otherwise leave a stale
     // tabs-auto-wrap class until the next content render.
     this.updateTabOverflowMode();
+    // Newly created tabs animate in; a re-render mid-cascade resumes them rather
+    // than restarting, since this rebuild just destroyed the animating elements.
+    this._applyTabEntrances?.();
   }
 
   // Set up arrow key navigation for session tabs (accessibility)
@@ -4100,6 +4116,10 @@ class CodemanApp {
     // selectSession or reconnect catches up.
     this._updateSseSubscription(sessionId);
     this.hideWelcome();
+    // Terminal-pane entrance: plays for a freshly created session, and on every
+    // switch when that option is on. Transform/opacity/clip-path only, xterm's
+    // FitAddon reads the untransformed layout box, so this cannot reach the PTY.
+    this.playTerminalEntrance?.(sessionId);
     // Clear idle hooks on view, but keep action hooks until user interacts
     this.clearPendingHooks(sessionId, 'idle_prompt');
     // Instant active-class toggle (no 100ms debounce), then schedule full render for badges/status