Integrate session state into the central state tier

[SBalaguer]

PR #21 hardened festival state into a single reactive tier (monotonic merge + pending overlay, fed by one watchBest() follow), but the wiring only covered the festival contract. Each FestivalSession is its own contract, so session-level state was left on the old per-component model. This caused four bugs, all from the same root:

Current state

• Joiner count was slow/stale. Nothing updated a session's registeredCount live (the festival watcher filters to one contract; useSessionWatcher didn't handle Registered). It moved only on a full reconcile.
• Badge animation misfired. Driven by a per-session follow that was fragile under the host follow budget (missed the event → didn't fire from the QR screen) and replay-sensitive (watchBest replays on resubscribe → fired twice on re-entry).
• Program didn't pull new sessions live. The single festival follow competed with per-session follows and stalled under budget pressure; new sessions appeared only on refresh.
• Session edits weren't reflected. handleUpdateMetadata wrote optimistically to a local ref only, never into shared

Changes

• One follow, dispatch by contract. Generalized the event watcher to route events to handlers keyed by emitting contract (the festival plus every session) over a single chainHead follow. Eliminates per-session follows and the follow-budget exhaustion.
• Session events through the merge tier. New applySessionRegistered / applySessionCheckedIn / applySessionMetadataUpdated helpers, replay-safe (registered count guarded by the unseen-row check, grounded by the reconcile's maxBig).
• Per-session, once-ever badge. Celebration now keys off shared check-in state plus a persisted per-session guard, so it fires once from the QR screen and never repeats on re-entry or reload.
• Session edits via the pending overlay. New editSession pending kind surfaces edits instantly across all views and rolls back on tx failure.
• It makes session time not editable, in line with contracts
• It re-directs users to session detail page after successful edit
• It fixes POAP reading upon checkin.

[github-actions]

Preview Deployment Ready!

App	Domain
Admin	pr30-manager
Festival	pr30-attendee

Deployment Details

App	CID
Admin	bafybeigc57q6fhvmxagaxkdgzzjgemoadptbsnvx7ssfstxogxueantd5a
Festival	bafybeiaj4atw6aju7kysbo2qnwcj42klfmptubgmezvrc6yc5heac2ra3u

Info	Value
Commit	797f7b0b54acf33db4ad43763027e53d96793e29
Branch	sb/fix/multiple-sessions-fix

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[tiagobndr]

LGTM!