Phase 6: Theme Unification

## Goal
Make Widgets and QML parts look consistent across light/dark/custom themes.

- `ThemeManager` QML_SINGLETON with canonical palette, synced with QPalette
- Refactor `Themed*.qml` components to use `ThemeManager` instead of `MaterialEditorQML`
- SVG icon set replacing current PNG icons for consistent scaling

**Depends on:** Phase 3 (QML components finalized)
**Part of:** UX Redesign Roadmap