fix: Fix invalid blocker state transition in UnsavedChangesProvider

Fix #1184: Time Unit error caused by invalid blocker state transition.

The error 'Invalid blocker state transition: unblocked -> proceeding' occurred
when calling blocker.proceed() in UnsavedChangesProvider.

Root cause: setDirty(false) was called before blocker.proceed?.(), which
triggered a re-render that could change the blocker state before proceed was
called. This caused React Router's internal state machine to receive a proceed
call when the blocker was no longer in the 'blocked' state.

Fix:
- Call blocker.proceed?.() before setDirty(false) in handleConfirm
- Add additional state check in isSamePathNavigation useEffect to ensure
  blocker is still 'blocked' before calling proceed()

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

frontend/src/components/UnsavedChangesProvider.tsx

@@ -72,8 +72,8 @@ export const UnsavedChangesProvider = ({
 
   const handleConfirm = useCallback(() => {
     if (blocker.state === 'blocked') {
-      setDirty(false)
       blocker.proceed?.()
+      setDirty(false)
     }
   }, [blocker, setDirty])
 
@@ -91,7 +91,7 @@ export const UnsavedChangesProvider = ({
 
   useEffect(() => {
     const proceed = blocker.proceed as (() => void) | undefined
-    if (isSamePathNavigation && proceed) {
+    if (isSamePathNavigation && proceed && blocker.state === 'blocked') {
       proceed()
     }
   }, [blocker, isSamePathNavigation])