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>

resources/icons/play-back.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+<path d="M 472 60 L 472 452 L 40 256 Z"/>
+</svg>