Timeline playback controls — transport icons, loop/bounce/rewind

- Replace all text buttons with SVG icon buttons (skip-back, step-back,
  play-back, play, pause, step-forward, skip-end)
- Add rewind: separate play-backward button that flips direction
- Add loop mode button with submenu (Loop / Bounce / Off)
  - Loop wraps at boundaries; Bounce reflects direction (ping-pong)
- Start/end frames moved to flank the scrub slider as plain numbers
- New icons: play-back.svg, loop.svg (FA repeat), bounce.svg (FA exchange-alt)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/IconManager.h

@@ -43,10 +43,13 @@ enum class Icon {
     // Timeline transport
     SkipBack,     // go to first frame  (|◀)
     StepBack,     // previous frame     (◀◀)
+    PlayBack,     // play in reverse    (◀)
     Play,         // play               (▶)
     Pause,        // pause              (⏸)
     StepForward,  // next frame         (▶▶)
     SkipEnd,      // go to last frame   (▶|)
+    Loop,         // loop toggle        (↻)
+    Bounce,       // bounce/ping-pong   (↔)
 };
 
 /// Loads SVG files from disk, rasterizes them with NanoSVG, uploads them as