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/bounce.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 5.15.4 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2021 Fonticons, Inc. --><path d="M0 168v-16c0-13.255 10.745-24 24-24h360V80c0-21.367 25.9-32 40.971-16.971l80 80c8.994 8.994 9.055 23.508 0 32.781l-80 80C409.956 271.748 384 261.641 384 240v-48H24c-13.255 0-24-10.745-24-24zm488 152H128v-48c0-21.591-25.993-31.806-40.971-16.971l-80 80c-9.055 9.273-8.994 23.787 0 32.781l80 80C102.1 463.93 128 453.686 128 432v-48h360c13.255 0 24-10.745 24-24v-16c0-13.255-10.745-24-24-24z"/></svg>

resources/icons/loop.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.7.2 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2024 Fonticons, Inc. --><path d="M0 224c0 17.7 14.3 32 32 32s32-14.3 32-32c0-53 43-96 96-96H320v32c0 12.9 7.8 24.6 19.8 29.6s25.7 2.2 34.9-6.9l64-64c12.5-12.5 12.5-32.8 0-45.3l-64-64c-9.2-9.2-22.9-11.9-34.9-6.9S320 19.1 320 32V64H160C71.6 64 0 135.6 0 224zm512 64c0-17.7-14.3-32-32-32s-32 14.3-32 32c0 53-43 96-96 96H192v-32c0-12.9-7.8-24.6-19.8-29.6s-25.7-2.2-34.9 6.9l-64 64c-12.5 12.5-12.5 32.8 0 45.3l64 64c9.2 9.2 22.9 11.9 34.9 6.9s19.8-16.6 19.8-29.6V448h160c88.4 0 160-71.6 160-160z"/></svg>

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>