docs(mobile): add comprehensive iOS build system documentation
Add detailed documentation for iOS build system learned through debugging:
- custom-protocol Cargo feature explanation (CRITICAL for production builds)
- Debug vs Release build separation (targets, caches, libraries)
- Why cargo build --lib instead of cargo tauri build (Swift FFI symbols)
- iOS cache system at repo root for workspace sharing
- Build scripts reference and workflows
- App Group containers and UUID lifecycle (when data persists vs lost)
- Troubleshooting guides: localhost error, builds not updating, wrong architecture
Also documents:
- Native WKWebView integration
- Pull-to-refresh with hold gesture
- In-app icon button styling
- Per-profile database files
- Cross-reference to docs/profiles.md
chore: remove accidentally committed backup and cache files
- Remove device-backup-20260204-143925/ directory (backup is in ~/sync/peek-backups/)
- Add device-backup-*/ to .gitignore to prevent future accidents
- Removes ~140 binary WebKit cache files that were accidentally committed
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