Settings toggle: hide sessions older than maxAge days

[Ron537]

Summary

Add a "Hide sessions older than N days" toggle in Settings → AI Tools that filters the Sessions panel.

Background

AppSettings.sessionMaxAgeDays already exists (default 30) but is currently used only for retention/garbage collection. The Sessions panel still shows everything regardless. Add a UI toggle that filters the in-app list to honor this same value.

Acceptance criteria

• New boolean setting hideSessionsOlderThanMaxAge defaulting to false (preserves current behavior).
• Toggle rendered in SettingsModal.tsx under the AI Tools tab, near the existing sessionMaxAgeDays slider/input.
• When enabled, SessionList filters out sessions whose updatedAt is older than now - sessionMaxAgeDays * 86400_000.
• Filter applies BEFORE search/grouping so counts stay consistent.

Files to touch

• src/renderer/src/types/index.ts
• src/renderer/src/stores/settingsStore.ts (default + migration)
• src/renderer/src/components/settings/SettingsModal.tsx
• src/renderer/src/components/sessions/SessionList.tsx (filter logic)

Effort: 3-5 hours.