fix: harden identifier validation, install warning, blob download, zoom repaint

- Validate custom --identifier format upfront with clear error message
- Warn user before overwriting existing app in /Applications via --install
- Add fetch() fallback in blob download when URL not in cache (fixes Gemini)
- Add reader.onerror handler to FileReader to surface silent failures
- Dispatch resize event after CSS zoom on macOS to fix rendering artifacts
- Prefix auto-generated identifier hash with 'a' to ensure D-Bus compliance on Linux