Show time since last change on dashboard widgets

[iTerminate]

Summary

• Add an opt-in per-slot dashboard widget option, show_last_changed, that shows a self-refreshing "Nm ago" line under a widget's state
• Available on every single-entity widget (sensor, binary_sensor, switch, light, fan, lock, cover, thermostat, media_player, weather, gauge); excluded from graph (already time-based) and panel/split (multi-entity)
• Toggled via a checkbox in the slot assignment popup, with live preview and prefill on reopen
• Persisted as a new show_last_changed column on dashboard_slots, migrated for existing DBs
• Demo dashboards seed a few examples (Front Door sensor, blinds, lock) with backdated timestamps

Closes #33

Test plan

• uv run pytest — full suite (1241 tests) passes
• uv run pyright — clean
• uv run ruff check . — clean
• Manually verified in uv run hatty --demo: Front Door widget on the Home dashboard shows "🚪 Closed / 4h ago"; the popup checkbox appears for a sensor slot and hides for graph/panel; reopening a slot with the flag set shows it pre-checked