Turn history retention into a discrete slider with named presets

[alvst]

The "remove clips older than" day picker offered a handful of choices and no way to say "keep everything". This replaces it with a 10-stop slider of named presets, adding 3 Weeks, 2 Months, and Forever.

What changed

• A new HistoryRetentionPreset drives a 10-stop slider from 1 Day through Forever, replacing the day-choice Picker.
• Forever is stored as historyRetentionDays == 0 — no automatic pruning by age; the existing safety cap still applies.
• The confirm-before-apply dialog for destructive changes is kept exactly as it was: only the presentation of the choice changes, not the safety UX around it.
• Layout fix at the call site: Form(.formStyle(.grouped)) puts every row in a Section's body onto a shared label/control NSGridView, which clamps and right-shifts a bare control regardless of SwiftUI frame modifiers. The slider block moved into the Section's footer, which is plain full-width content.

Screenshots

Before:

After:

Notes for review

• Stored-value migration is safe: every previously stored day count remains a valid stop, so existing installs land on their current setting.
• The confirm dialog still covers every destructive direction — mode switch, tighter preset, and Forever → short.
• No dependencies; merges in any order.

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This feature should be bundled into v2.

Part of #80.