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)

.github/workflows/ci.yml

@@ -25,6 +25,9 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
+      - name: Lint
+        run: npm run lint
+
       - name: Type check
         run: npx tsc --noEmit