fix: prevent clipboard restore from running when task is cancelled

[Ahmet-4955]

Summary

Replace try? with do/catch { return } in schedulePasteboardRestore to correctly handle Swift Task cancellation.

Problem

try? on Task.sleep silently swallows CancellationError, causing the restore task to continue executing immediately after being cancelled — resulting in the old clipboard content being pasted instead of the transcribed text.

Race condition:

1. Transcript written to clipboard with marker M
2. 60 s fallback restore task (A) scheduled with marker M
3. performPaste() fires Cmd+V
4. 1.5 s post-paste restore task (B) scheduled → cancels task A
5. Task A: try? swallows CancellationError → task continues running immediately
6. Task A: marker M still matches → old clipboard content restored
7. Target app processes Cmd+V — but clipboard now has old content → wrong text pasted

Fix

// Before
try? await Task.sleep(for: .seconds(delay))

// After
do {
    try await Task.sleep(for: .seconds(delay))
} catch {
    return
}

Cancellation now causes the task to exit cleanly without triggering the clipboard restore.

Test plan

• Record audio via Blitztext popover or hotkey
• Confirm the transcript — not the previous clipboard content — is pasted into the target app
• Confirm the previous clipboard content is correctly restored ~1.5 s after the paste (press Cmd+V again to verify)

🤖 Generated with Claude Code