fix: clearAll() leaks audio files (#101)

[karansinghgit]

Fixes #101.

Problem

HistoryService.clearAll() removed transcript items but left their backing audio files on disk forever, so "Clear All" silently leaked storage:

func clearAll() {
    items.removeAll()
    saveHistory()
}

Fix

Delete each item's audio file before clearing, reusing the existing removeAudioFileIfNeeded(for:) — the same thing deleteItem does by default.

Deliberately not changed: stats

The issue also expected statsEntries to reset. I left stats intact on purpose: the Clear All confirmation dialog explicitly says "This removes your saved transcripts, but keeps your statistics history." Resetting stats would contradict that documented promise. If you'd actually prefer Clear All to wipe stats too, that's a product decision — say the word and I'll update both the method and the dialog copy in a follow-up.

Test plan

• Record a few transcriptions (creates audio files) → Clear All → audio files are gone from Application Support; stats history remains.