refactor(studio): code quality — 22 findings, dead code removal, App.tsx split (#144)

## Summary

Full code quality review of the studio package, fixing 22 of 25 findings. Removes dead code, extracts modules from App.tsx, fixes accessibility and performance issues.

## Critical fixes (3)

- **`aria-valuenow`** on seek bar now updates imperatively via `liveTime.subscribe` — screen readers previously always reported position 0
- **Speed menu** closes on outside click (was permanently stuck open)
- **RenderQueue auto-scroll** moved from render phase to `useEffect` (was violating React render purity via `queueMicrotask` during render)

## Dead code removed (-331 lines)

| File | Lines | Why dead |
|---|---|---|
| `PreviewPanel.tsx` | 180 | Replaced by NLELayout + NLEPreview |
| `useCodeEditor.ts` | 80 | Exported but never imported |
| `formatTick` alias | 2 | Deprecated, unused |
| `onClipChange` prop | 5 | Declared, never used |
| `trackH` prop | 5 | Declared, never used |
| `editRange*` + updaters in store | 60 | Never read or written |

## App.tsx extraction

| Extracted to | Lines | What |
|---|---|---|
| `components/LintModal.tsx` | 130 | Lint results modal + LintFinding type |
| `components/MediaPreview.tsx` | 75 | Image/video/audio/font file previewer |
| `utils/mediaTypes.ts` | 15 | Shared regex constants (App.tsx and AssetsTab.tsx had diverged copies) |

## Performance fixes

- `useMemo` for `compositions`/`assets` derivation from `fileTree`
- `useMemo` for `buildTree(files)` in FileTree
- Debounced `handleContentChange` PUT (600ms — was firing on every keystroke)
- CompositionsTab iframe hover debounced (300ms — was mounting immediately)
- `VideoFrameThumbnail` re-extracts frame when `src` prop changes

## Not addressed (3 — low priority)

- #6: SystemIcons consolidation (large refactor across many files)
- #16-17: Overlay dismiss pattern standardization
- #18: Inline SVG → Phosphor replacement (gradual, per-PR)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

packages/studio/src/components/ui/VideoFrameThumbnail.tsx

@@ -1,4 +1,4 @@
-import { useState, useEffect, useRef } from "react";
+import { useState, useEffect } from "react";
 
 /**
  * Extracts a representative JPEG frame from a video URL using a hidden
@@ -7,12 +7,8 @@ import { useState, useEffect, useRef } from "react";
  */
 export function VideoFrameThumbnail({ src }: { src: string }) {
   const [frame, setFrame] = useState<string | null>(null);
-  const didExtract = useRef(false);
 
   useEffect(() => {
-    if (didExtract.current) return;
-    didExtract.current = true;
-
     const video = document.createElement("video");
     video.crossOrigin = "anonymous";
     video.muted = true;