Allow any list to be designated as a notification list

[iTerminate]

Summary

• Replaces the single hardcoded reserved 🔔 Notifications list with a notify_lists set: any user list (or several) can be toggled as a change-alert source, from the same list popup used to rename/delete lists (n binding).
• The enabled notification preference is now a pure global mute — it no longer drives list visibility, since there's no reserved list left to hide.
• A one-time storage migration carries a pre-existing reserved list's entities forward as an ordinary designated list (or drops it if it was never used).
• Config screen's Notifications pane now summarizes the designated lists + total watched-entity count, and "Clear watched entities" is now "Stop watching all lists" (clears designations only, never list contents).
• Demo mode swaps the old seeded reserved list for a curated "Security" list pre-designated as a notify source.

Test plan

• pytest (full suite, 1218 tests) passes
• ruff check . clean
• pyright clean (0 errors)
• Manually verified in --demo: list popup marks designated lists with 🔔, n toggles in place with a confirmation toast, and the designation survives a list rename

Refs #24