chore: bump to v0.9.19-rc.1 and add local OpenAPI build support

- Add release candidate (rc) support to release.sh with latestRC tagging
- Bump VERSION, backend pyproject.toml, and frontend package.json to 0.9.19-rc.1
- Add local frontend/openapi.json so build no longer needs running backend
- Update generate:types and validate-types.sh to use local openapi.json
- Fix frontend tests: remove unused imports/variables and update mock data

frontend/src/hooks/__tests__/usePolledData.test.ts

@@ -34,8 +34,6 @@ describe("usePolledData", () => {
       vi.runAllTimersAsync();
     });
 
-    const callCountAfterInitial = fetcher.mock.calls.length;
-
     // Reset timer and advance to ensure no more polls
     vi.clearAllTimers();
     fetcher.mockClear();
@@ -66,8 +64,6 @@ describe("usePolledData", () => {
       vi.advanceTimersByTime(100);
     });
 
-    const initialCalls = fetcher.mock.calls.length;
-
     // Clear for clean test
     fetcher.mockClear();
 
@@ -135,7 +131,6 @@ describe("usePolledData", () => {
       vi.advanceTimersByTime(100);
     });
 
-    const initialCalls = fetcher.mock.calls.length;
     fetcher.mockClear();
 
     // Call refresh