feat(theme): add visual sidebar overrides

[RunningCoderLee]

Summary

• add visual-only sidebar labels, navigation icons, project-folder icons, and composer placeholder overrides for local macOS themes
• keep native text, accessibility labels, event handlers, project/task names, ordering, and expansion state unchanged
• stage referenced PNG assets atomically with the theme and include them in the runtime content fingerprint
• keep the canonical renderer attribute-only and synchronize the generated macOS/Windows runtime assets

Why

The existing background theme contract could not preserve a complete visual pack when switching themes. Custom navigation and project-folder artwork had to be applied separately, and the pre-v1.5 implementation depended on retired renderer marker classes.

Validation

• node --test macos/tests/*.test.mjs windows/tests/*.test.mjs — 76 passed
• macOS complete repository regression suite — passed
• macOS Swift build and XCTest — 10 passed
• both platform payload checks — passed
• shared runtime asset synchronization and git diff --check — passed

Signed-runtime integrations and Doctor were skipped using the same repository test mode as CI because they require an installed signed app/runtime.

Scope notes

• generated images and screenshots under output/ are intentionally not included
• official/simple ZIP import contracts remain unchanged; visual icon packs are currently supported in local macOS saved theme folders
• no version bump or release publication is included