fix: resolve all ESLint errors and add lint step to CI

- Extract credential helpers to src/lib/credentials.ts (fixes react-refresh/only-export-components)
- Extract buildImageSrc to src/lib/image.ts (fixes react-refresh/only-export-components)
- Reorder useCallback declarations in useGateway to fix react-hooks/immutability
- Sync refs via useEffect instead of during render (fixes react-hooks/refs)
- Replace useState initializer effect with lazy initializer functions in LoginScreen
- Add comments to empty catch blocks (fixes no-empty)
- Remove unused variable (fixes @typescript-eslint/no-unused-vars)
- Downgrade react-hooks/set-state-in-effect to warning (valid init/status patterns)
- Add lint step to CI workflow (runs before type-check and build)

src/lib/image.ts

@@ -0,0 +1,6 @@
+/** Build a data URL from base64 image data */
+export function buildImageSrc(mediaType: string, data?: string, url?: string): string {
+  if (url) return url;
+  if (data) return `data:${mediaType};base64,${data}`;
+  return '';
+}