feat(studio): timeline UI overhaul — flat clips, unified color, working thumbnails

Visual redesign of the timeline:
- Flat solid clip backgrounds, no gradients or multi-layer shadows
- Unified neutral color palette — all clips use the same base color
- Clean 6px border-radius instead of organic asymmetric radii
- Single-line labels, no redundant tag badge or time range
- 3px teal accent stripe on the left edge of every clip
- Simplified trim handles (2px accent bars)

Thumbnail fixes:
- Fixed broken thumbnail URLs — compositionSrc was an absolute URL
  that got nested inside the preview/comp path. Now normalized to
  relative path before constructing the thumbnail URL
- Thumbnail background changed from #000 to #1c2028 so transparent
  overlay compositions render visible content against a matching
  dark background
- mix-blend-mode: lighten on thumbnail layer — dark backgrounds blend
  away, bright content shows through
- Removed double-label in CompositionThumbnail (was showing both a
  badge at top and text at bottom)

packages/cli/src/server/studioServer.ts

@@ -349,7 +349,13 @@ export function createStudioServer(options: StudioServerOptions): StudioServer {
         }, opts.seekTime);
         const manifestContent = readStudioManualEditManifestContent(opts.project.dir);
         await applyStudioManualEditsToThumbnailPage(page, manifestContent, opts.compPath);
-        await page.evaluate(() => document.fonts?.ready);
+        await page.evaluate(() => {
+          void document.fonts?.ready;
+          const body = document.body;
+          if (body && getComputedStyle(body).backgroundColor === "rgba(0, 0, 0, 0)") {
+            body.style.backgroundColor = "#1c2028";
+          }
+        });
         await new Promise((r) => setTimeout(r, 200));
         await reapplyStudioManualEditsToThumbnailPage(page);
         let clip: ScreenshotClip | undefined;