Add time-based history retention ("keep for N days")

[danielsyauqi]

Summary

Implements time-based history retention, as discussed in #368. A new optional setting, Keep for (historyKeepDays), automatically removes unpinned history items whose last copy is older than a configured number of days. Pinned items are never removed.

The setting defaults to 0, which disables the time-based limit, so existing behaviour is unchanged unless the user opts in.

Motivation

Per the discussion in #368, the maintainer noted that a time-based limit was desirable but should follow the in-memory storage work (#384). Because this implementation deletes expired items from the SwiftData store, it reduces the number of items loaded into memory over long-running sessions rather than adding to the footprint, so it also acts as a partial mitigation for the memory growth reported in #384 and #1240.

To keep the first iteration simple, format-specific retention (separate limits for text vs. images) is intentionally left out; it can be layered on later if there is demand.

Changes

• Defaults.Keys.historyKeepDays (Int, default 0 = disabled).
• History.purgeExpiredItems() removes unpinned items with lastCopiedAt older than the cutoff. It is invoked:
  • on launch (History.load()),
  • after each new copy (History.add()),
  • when the setting changes, and
  • on an hourly timer, so the limit is enforced while the app stays open and idle (the case the count-based limit cannot cover).
• A Keep for control in the Storage settings pane, alongside Size, showing "Disabled" when set to 0.
• English strings for the new control.
• Unit tests covering removal of expired items, preservation of recent and pinned items, and the disabled (0) case.

Testing

• xcodebuild ... build succeeds.
• The new tests pass when run directly.
• The full MaccyTests run shows no new failures introduced by this change.

Notes for reviewers

• The new tests are added to HistoryTests, which is currently listed under skippedTests in Maccy.xctestplan; they therefore will not run in CI as-is. They pass when the class is run directly. Happy to adjust the test plan if preferred.
• Only English (en.lproj) strings are included. The other localizations fall back to English until translated.