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/components/ImageBlock.tsx

@@ -66,9 +66,4 @@ export function ImageBlock({ src, alt }: ImageBlockProps) {
   );
 }
 
-/** 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 '';
-}
+// buildImageSrc moved to ../lib/image.ts