Revoke previous recording preview URL before starting a new take

[AlexMnrs]

Problem

PromptFlow creates an object URL for each recorded take so the user can review, download, or share it in the browser. The current URL is revoked when a recording finishes and replaces an existing review URL, and also when the user dismisses the take.

There is one gap: starting a new recording clears recordedUrl with setRecordedUrl('') before the next recorder.onstop callback gets a chance to revoke the previous URL. That can leave the old blob: URL alive until the page reloads.

Relevant code: src/App.tsx, around the beginRecording, dismissRecording, and recording review state.

Proposed fix

Revoke the existing recorded take URL before clearing it when a new recording starts. Keep the existing dismiss behavior intact.

Acceptance criteria

• Starting a new recording revokes any previous recorded take object URL.
• Dismissing a take still revokes the current object URL.
• Repeated record/review/record cycles do not leave stale object URLs behind.
• Download and share behavior for the latest take still works.

Verification

• Run npm run lint.
• Run npm test.
• Run npm run build.
• If possible, manually record two short takes in a row and confirm the latest take can still be reviewed/downloaded.