Add edit RSVP modal, plus-one tracking, and unified signup form

Merge RSVP + slot claim into a single form. Add plus_one field to RSVPs.
Add clickable RSVP names that open a modal to edit name/note/plus_one.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

schema.sql

@@ -33,6 +33,7 @@ CREATE TABLE IF NOT EXISTS rsvps (
     event_id   INTEGER NOT NULL REFERENCES events(id) ON DELETE CASCADE,
     name       TEXT    NOT NULL,
     note       TEXT    NOT NULL DEFAULT '',
+    plus_one   INTEGER NOT NULL DEFAULT 0,
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 );