feat: lens effects tab, standalone motion effects, clip/asset rename, preview popout

- New Lens category in the assets panel: Blur, Motion Blur, Zoom In/Out,
  Pan x4, Shake, Pulse, Spin, Custom Zoom. EffectDefinition gains a
  category field; tabs with an empty category are hidden, and the
  properties-panel 'Open effects' button opens the first non-empty one.
- Motion effects work standalone: dropping them on the timeline creates
  an effect-track element that animates (scale/translate/rotate) every
  track below it, computed purely in the TS frame descriptor - no wasm
  changes, identical in preview and export. supportsStandalone removed.
- Motion Blur: directional gaussian smear (amount + angle) reusing the
  existing WGSL shader with arbitrary u_direction.
- Custom Zoom: start/end scale params (keyframable clip-level).
- MotionOffset gains optional rotate, accumulated clip-level and applied
  to standalone layer quads via rotationDegrees.
- Rename clips (timeline context menu) and media assets (media panel
  context menu) via a shared RenameDialog; asset rename persists through
  storageService with rollback on error.
- Preview pops out into a Document Picture-in-Picture window and docks
  back; button hidden where the API is unsupported.
- Export keeps running when the export popover is closed; only the
  explicit Cancel button aborts.
- Captions and Adjustment tabs removed from the assets panel.

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