Flash tab title when the doorbell rings

[TrystonPerry]

Summary

• When /doorbell receives a ring, cycle document.title every second between the default title and Someone rang the doorbell.
• Show an ack dialog (existing Modal + Button) so an organizer looking at the page can dismiss it.
• Stop the title cycle when the tab is back in view, or when the dialog is dismissed (Got it, X, Escape, backdrop). Coming back to the tab only stops the flash; the dialog stays until they click it.

This is the operator cue for a background doorbell tab. Guest QR flow is unchanged.

Test plan

• Open /doorbell and click Ring Doorbell. Confirm the dialog appears and the tab title flips every second.
• Click Got it (or X / Escape / backdrop). Confirm the title restores and the dialog closes.
• Ring again, switch to another tab, confirm the title still cycles, then switch back. Confirm the title stops cycling and the dialog is still open until dismissed.
• Open two tabs on /doorbell, ring in one, confirm the other tab also alerts (Supabase Realtime).