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/credentials.ts

@@ -0,0 +1,21 @@
+const STORAGE_KEY = 'pinchchat_credentials';
+
+export function getStoredCredentials(): { url: string; token: string } | null {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY);
+    if (!raw) return null;
+    const parsed = JSON.parse(raw);
+    if (parsed.url && parsed.token) return parsed;
+  } catch {
+    // Ignore malformed localStorage data
+  }
+  return null;
+}
+
+export function storeCredentials(url: string, token: string) {
+  localStorage.setItem(STORAGE_KEY, JSON.stringify({ url, token }));
+}
+
+export function clearCredentials() {
+  localStorage.removeItem(STORAGE_KEY);
+}