fix(ui): Sprint 9 — dismiss X / Skip tour did not hide the dialog

Root cause: the OnboardingTour early-return is gated on
isComplete=true, but App.tsx was calling onboarding.reset() on
onComplete. reset() does the inverse: clears the localStorage key
and flips isComplete to FALSE. The user clicked X, the localStorage
key got written, but the App-level flag flipped to false, so the
tour re-rendered and the early-return did not fire — the dialog
stayed visible.

Fix: split the dismiss and reset paths into two distinct callbacks
onComplete (dismiss) and onReset (re-show). Added markComplete to
useOnboarding: flips isComplete to true. App wires:
  onComplete -> onboarding.markComplete()
  onReset    -> onboarding.reset()
The tour itself still calls writeComplete() before invoking
onComplete, so the localStorage key is written once on dismiss.

Also cleaned markComplete: it now only flips state (the tour already
wrote the key), removing a redundant double-write.

Verified npm run build green on docker-willester. No regression
expected; all other Sprint 9 code paths untouched.

frontend/src/App.tsx

@@ -102,9 +102,16 @@ function App() {
             <OnboardingTour
               isComplete={onboarding.isComplete}
               onComplete={() => {
-                // The tour already wrote the localStorage key via
-                // writeComplete(); flip the App-level flag so it stays
-                // hidden after the next render.
+                // Dismiss path (X / Skip / Esc / "Got it"). The tour
+                // already wrote the localStorage key; flip the
+                // App-level flag to true so the early-return fires and
+                // the dialog disappears.
+                onboarding.markComplete()
+              }}
+              onReset={() => {
+                // Re-show path (?reset-tour=1). The tour already
+                // cleared the localStorage key; flip the App-level
+                // flag to false so the tour re-appears.
                 onboarding.reset()
               }}
             />