feat: implement Content Audit tool and Tabbed Navigation

### Context
Introduce the "Audit" feature to flag "Ugly HTML" (Figma bloat, MS Word bloat, missing Alt tags). This warrants the move to a Tabbed UI to separate Search from Auditing.

### Tasks
- [ ] Build `src/core/audit/` logic to flag Figma/Word artifacts and missing `alt` tags.
- [ ] Implement Tabbed UI (Search, Audit, Settings).
- [ ] Build the Results View for the Audit tool.