Fix all 135 vue-tsc errors, promote to check gate
vue-tsc now runs as part of 'bun run check' — zero errors.
Fixes by category:
- TS18047 (73): null guards in LayoutSection, TypographySection
- TS18048 (17): collab injection guards in CollabPanel, MobileHud
- TS2339 (12): Uint8Array.fromBase64/toBase64 declarations (ES2024)
- TS2741 (10): AppMenu discriminated union for separators
- TS6133 (7): remove unused declarations
- TS2322 (6): FontPicker AcceptableValue, Toolbar motion-v variants
- TS2345 (3): FillSection updateArrayItem signature, ScrubInput guard
- TS2314 (2): ChatMessage UIMessagePart generic args
- Misc (5): PropertiesPanel dead comparison, VariablesDialog onSubmit,
NodeContextMenuContent document access, LayerTree arg count
4b49d69601