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/renders/RenderQueue.tsx

@@ -1,9 +1,10 @@
-import { memo, useState, useRef } from "react";
+import { memo, useState, useRef, useEffect } from "react";
 import { RenderQueueItem } from "./RenderQueueItem";
 import type { RenderJob } from "./useRenderQueue";
 
 interface RenderQueueProps {
   jobs: RenderJob[];
+  projectId: string;
   onDelete: (jobId: string) => void;
   onClearCompleted: () => void;
   onStartRender: (format: "mp4" | "webm") => void;
@@ -33,7 +34,7 @@ function FormatExportButton({
       <button
         onClick={() => onStartRender(format)}
         disabled={isRendering}
-        className="flex items-center gap-1 px-2 py-0.5 text-[10px] font-semibold rounded-r bg-[#3CE6AC] text-[#09090B] hover:brightness-110 transition-colors disabled:opacity-50"
+        className="flex items-center gap-1 px-2 py-0.5 text-[10px] font-semibold rounded-r bg-studio-accent text-[#09090B] hover:brightness-110 transition-colors disabled:opacity-50"
       >
         {isRendering ? "Rendering..." : "Export"}
       </button>
@@ -43,21 +44,21 @@ function FormatExportButton({
 
 export const RenderQueue = memo(function RenderQueue({
   jobs,
+  projectId,
   onDelete,
   onClearCompleted,
   onStartRender,
   isRendering,
 }: RenderQueueProps) {
   const listRef = useRef<HTMLDivElement>(null);
-  const prevCount = useRef(jobs.length);
 
-  // Auto-scroll to bottom when new jobs are added (adjust during render)
-  if (jobs.length > prevCount.current && listRef.current) {
-    queueMicrotask(() => {
-      listRef.current?.scrollTo({ top: listRef.current.scrollHeight, behavior: "smooth" });
-    });
-  }
-  prevCount.current = jobs.length;
+  // Auto-scroll to bottom when new jobs are added.
+  // Runs in an effect to avoid side effects during the render phase.
+  useEffect(() => {
+    if (listRef.current) {
+      listRef.current.scrollTo({ top: listRef.current.scrollHeight, behavior: "smooth" });
+    }
+  }, [jobs.length]);
 
   const completedCount = jobs.filter((j) => j.status !== "rendering").length;
 
@@ -111,7 +112,12 @@ export const RenderQueue = memo(function RenderQueue({
           </div>
         ) : (
           jobs.map((job) => (
-            <RenderQueueItem key={job.id} job={job} onDelete={() => onDelete(job.id)} />
+            <RenderQueueItem
+              key={job.id}
+              job={job}
+              projectId={projectId}
+              onDelete={() => onDelete(job.id)}
+            />
           ))
         )}
       </div>