feat(history): add automatic transcription history clearing (#463)

[devzahirul]

Summary

Implements #463: an Automatically Clear History setting so users who dictate sensitive content don't have to remember to clear transcription history manually.

Options: Never (default, current behavior), End of Day, After 7 Days, After 30 Days, After 90 Days.

Design

The issue proposes scheduled wipes ("cleared daily at midnight"). Since the app can't rely on being awake at a scheduled moment, this is implemented as retention-based pruning — entries older than the selected window are deleted whenever the store gets a chance:

• at store load (app launch),
• before each new entry is added,
• at midnight while running, via NSCalendarDayChanged (so "End of Day" behaves exactly as the issue describes for long-running sessions),
• immediately when the user tightens the interval.

Cutoffs are anchored to startOfDay, so a retained day never partially expires (e.g. "After 7 Days" keeps 7 full calendar days plus today). Pruned entries also delete their saved dictation audio via DictationAudioHistoryStore, and the sidebar selection is repaired if the selected entry expires.

Safety

• Default is Never — zero behavior change unless the user opts in.
• If changing the setting would immediately delete existing entries, a confirmation alert (matching the existing audio-budget prune flow) states how many entries are affected and that stats/streaks based on them will change. Cancel leaves the setting untouched.
• The row is disabled while Save Transcription History is off, matching the adjacent audio toggle.
• Unknown persisted values fall back to Never.

Changes

• SettingsStore — new HistoryAutoClearInterval enum (display name, help text, cutoffDate) and persisted historyAutoClearInterval setting.
• TranscriptionHistoryStore — new pruneExpiredEntries(); called from init, addEntry, and a calendar-day-change observer.
• SettingsView — picker row under the history toggles with the confirmation flow described above.
• Tests — HistoryAutoClearIntervalTests covering the no-cutoff case, end-of-day boundary (today survives, yesterday expires), start-of-day anchoring for the rolling windows, and raw-value fallback.

Testing

• swiftlint --strict — 0 violations.
• xcodebuild build (Fluid scheme) — succeeds.
• xcodebuild test — full suite green; the 5 new tests pass.

Closes #463

[chatgpt-codex-connector]

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