chore: stage all pending work — caption styling, media server, processing modal, docs, summaries

Includes:
- Extended caption styling (font, shadow, dimmed color, bg toggle)
- Media server, subtitle downloader, VTT parser, processing modal
- Waveform tiers, thumbnail/timeline improvements, transport controls
- Hybrid download model, dependency management, clip export enhancements
- 21 chat summaries, 2 implementation plans, 2 design specs

Co-authored-by: Cursor <cursoragent@cursor.com>

src/lib/timeline/interactions.ts

@@ -45,6 +45,28 @@ export function computeZoom(
   return { visibleStart: newStart, visibleEnd: newEnd, zoom: newZoom };
 }
 
+export function panBy(
+  state: TimelineState,
+  deltaPixels: number,
+  duration: number
+): { visibleStart: number; visibleEnd: number } {
+  const range = state.visibleEnd - state.visibleStart;
+  const deltaTime = (deltaPixels / state.width) * range;
+  let newStart = state.visibleStart + deltaTime;
+  let newEnd = newStart + range;
+
+  if (newStart < 0) {
+    newStart = 0;
+    newEnd = range;
+  }
+  if (newEnd > duration) {
+    newEnd = duration;
+    newStart = Math.max(0, newEnd - range);
+  }
+
+  return { visibleStart: newStart, visibleEnd: newEnd };
+}
+
 export function handleClick(
   x: number,
   state: TimelineState,