fix(bootstrap): treat leaving failed as a new attempt

Review finding on 5e9538a0. The reset keyed only off arriving at a restart
stage, so a retry whose restart stage the poll never sampled did not reset at
all: failed -> checking -> starting_backend inside one ~1s window surfaces as
failed -> starting_backend, leaving the FAILED attempt's stages in
polledStages and rendering its install chrome as this attempt's completed
work. Not merely a late boundary — no boundary.

retry_bootstrap / clean_and_retry_bootstrap are the only exits from `failed`,
so leaving that stage is itself proof a new attempt began. Keying off it as
well as off restart stages closes the case without new producer state.

Regression test fails against 5e9538a0, passes here. Suite green (25 tests).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/components/BootstrapSplash.jsx

@@ -568,7 +568,17 @@ export function BootstrapSplash({ stage, message }) {
   useEffect(() => {
     const prev = prevStageRef.current;
     prevStageRef.current = stage;
-    const restarted = RESTART_STAGES.has(stage) && !RESTART_STAGES.has(prev);
+    // Two ways a new attempt shows up in the poll. Arriving at a restart
+    // stage is the common one. But the poll can also miss the restart stage
+    // entirely — `failed` -> (retry) -> `checking` -> `starting_backend`
+    // inside one ~1s sample window surfaces as `failed` -> `starting_backend`,
+    // and keying only off restart stages would leave the FAILED attempt's
+    // evidence in place and render its install chrome as this attempt's
+    // completed work. Leaving `failed` at all means a retry began, since
+    // retry_bootstrap/clean_and_retry_bootstrap are the only exits from it.
+    const restarted =
+      (RESTART_STAGES.has(stage) && !RESTART_STAGES.has(prev)) ||
+      (prev === 'failed' && stage !== 'failed');
     if (restarted) {
       if (selfInitiatedRef.current) {
         // beginAttempt() already opened this attempt with an exact boundary.

frontend/src/test/BootstrapSplashObservedStages.test.jsx

@@ -205,4 +205,25 @@ describe('BootstrapSplash — observed-stage tracking (#1894)', () => {
       expect(venvStep.className).toMatch(/text-fg-muted/);
     });
   });
+
+  it('a retry whose restart stage the poll never sampled still drops the old evidence', () => {
+    // CodeRabbit finding on 5e9538a0: a retry can go failed -> checking ->
+    // starting_backend inside one ~1s sample window, so the poll observes
+    // only failed -> starting_backend. Keying the reset solely off arriving
+    // at a restart stage would leave the FAILED attempt's stages in place and
+    // present them as this attempt's completed work.
+    const { rerender } = render(<BootstrapSplash stage="checking" message={null} />);
+    rerender(<BootstrapSplash stage="downloading_uv" message={null} />);
+    rerender(<BootstrapSplash stage="installing_deps" message={null} />);
+    rerender(<BootstrapSplash stage="failed" message="uv sync failed" />);
+
+    // Retry — and the poll misses `checking` entirely.
+    rerender(<BootstrapSplash stage="starting_backend" message={null} />);
+
+    expect(screen.getByText('Starting backend…')).toBeInTheDocument();
+    // Nothing from the failed attempt may be shown as this attempt's work.
+    expect(screen.queryByText('Downloading uv (Python package manager)…')).toBeNull();
+    expect(screen.queryByText(/first run, 5.10 min/)).toBeNull();
+    expect(screen.queryByText('Installing')).toBeNull();
+  });
 });