feat: improve grace period logic to pick up recently ended sessions

docs: update guide and README to reflect always-visible overlay and 2h grace period

feat: keep overlay 2 hours after session ends before clearing

fix: improve hr visibility and Next Event label size in schedule list

feat: overlay stays visible, clear on session end, add DB rules, bump Firebase SDK to 11.6.0

Reset ceramah_overlay on schedule reset — clears name, title, hides overlay, nulls lastSessionKey

Reorder guide.html sections — move What to Do When to position 2

Update README with data structure and features; make guide.html mobile responsive

Persist lastSessionKey in Firebase to fix session-end detection after page reload

Fix overlay not hiding after session ends — detect transition via lastActiveSessionKey

Cleanup: remove unused font/variable/class, deduplicate parseDateTime, clear stale toast text

Overlay UI: replace SVG arrow with image, equalise top/bottom bar heights

Rewrite guide.html for end users — no technical jargon

Option B: respect isVisible during gaps, single source of truth, add guide.html, help link in control

Fix overlay session sync: auto-reset isVisible on new session, write back to Firebase, reduce poll to 10s