fix(studio): iPhone Safari layout + touch-drag scrubber (#308)

Stacked on top of #307. Fixes three mobile UX bugs that made the studio unusable on iPhone Safari — discovered while testing the audio-ownership work from #307 on a physical device.

## Bugs fixed

### 1. Untappable Play button / bottom controls

`#root` was set to `100vh`. iOS Safari reports `100vh` as the **largest** viewport (toolbar hidden) and never shrinks it — so with the toolbar visible, the bottom of the layout sits under it. The Play button + timecode were fully occluded.

### 2. Scrubber not draggable by touch

The seek bar had only `onMouseDown`. Mouse events don't fire for touches on iOS Safari, so nothing responded. You could tap to jump but not drag.

### 3. Safari's horizontal swipe hijacked scrubber drags

Even when the seek bar caught `pointerdown`, `touch-action: manipulation` still let Safari consume horizontal edge-swipes for back-navigation — dragging the scrubber left was impossible.

## What changed

| File | Fix |
|---|---|
| `packages/studio/src/styles/studio.css` | `#root` → `height: 100dvh` with `100vh` fallback. Dynamic viewport height shrinks when the iOS toolbar is visible, so the bottom of `#root` lines up with the visible area. |
| `packages/studio/src/App.tsx` | Two `h-screen` containers → `h-full` so nested children fill the now-dynamic parent instead of asserting `100vh` and overflowing. |
| `packages/studio/index.html` | Added `viewport-fit=cover` so iOS exposes real `env(safe-area-inset-bottom)` values. |
| `packages/studio/src/player/components/PlayerControls.tsx` | Controls row gets `padding-bottom: calc(0.5rem + env(safe-area-inset-bottom))` so it clears the landscape home indicator. Scrubber replaced `onMouseDown` with `onPointerDown` + `setPointerCapture`, plus `touch-action: none` so Safari doesn't hijack horizontal swipes. Added `pointercancel` + window-level `pointerup` fallbacks. |

All desktop code paths are unchanged: `100dvh` falls back to `100vh`, `env(safe-area-inset-bottom)` is `0` off-iOS, Pointer Events subsume Mouse Events on desktop.

## Verified live

Via the `cloudflared` tunnel I ran during review on the factory-series-c-video project:

- iPhone Safari, portrait: Play button now fully visible and tappable. Bottom controls sit just above the URL bar.
- iPhone Safari, landscape: controls clear the home indicator.
- Finger-drag the scrubber left and right: tracks the touch smoothly, finger can leave the 6 px bar height without losing the drag.
- Desktop click-to-seek and click-drag: still work.
- Arrow-key seeking: still works.

## Stacked dependency

Base is `fix/player-audio-ownership-review` (PR #307). Once #307 merges, rebase this branch onto `main` — the changes are fully independent; the stacking is just to avoid waiting on the review for #307 before shipping pure UX wins.

## Test plan

- [x] `tsc --noEmit` on `packages/studio` — clean
- [x] `bun run --filter @hyperframes/studio build` — clean
- [x] Live repro on iPhone Safari via cloudflared tunnel: Play button tappable, scrubber drags with touch
- [ ] Android Chrome sanity pass before release (same Pointer Events code path, but worth eye-balling)

packages/studio/src/styles/studio.css

@@ -13,7 +13,18 @@ body {
 
 #root {
   width: 100vw;
+  /*
+   * 100vh on iOS Safari measures the LARGEST viewport (toolbars hidden) and
+   * stays fixed at that value, so when the toolbar is visible the bottom of
+   * the layout sits *under* it and anything at flex-end — the player
+   * controls row, notably — becomes untappable. `100dvh` follows the
+   * dynamic viewport, shrinking when the toolbar is shown so the bottom of
+   * #root lines up with the bottom of the visible area. Fallback to 100vh
+   * keeps older browsers (pre-Safari 15.4 / Firefox 101 / Chrome 108) on
+   * the existing behaviour.
+   */
   height: 100vh;
+  height: 100dvh;
 }
 
 /* CodeMirror overrides */