feat: streak-loss reminder notification

[code418]

What & why

Expands the notification system with the first gameplay push (the existing ones are all social). Once a day, at an unpredictable time between 5pm and 9pm UK time, we remind players who claimed yesterday but haven't claimed today that they'll lose their daily streak unless they claim before midnight.

How it works

New scheduled function streakReminder (functions/src/streakReminder.ts), mirroring the newDayScoreboard pattern (thin onSchedule wrapper + exported, unit-tested helpers):

• Cloud Scheduler can't fire at a random time, so the function polls every 15 min (*/15 17-20 * * *, Europe/London) across a 16-slot window (17:00 … 20:45). On the day's first run it rolls a random target slot and fires once when that slot is reached — giving an unpredictable send time that's guaranteed to land inside the 5–9pm window. Firing on slot >= target (not ==) keeps it robust to a skipped scheduler run.
• Dispatch state ({ day, targetSlot, sentDay }) lives in a server-only notificationState/streakReminder doc, read-and-written inside a transaction so two overlapping invocations can't double-send.
• Recipients: a single indexed equality query users where lastClaimDate == yesterday — since lastClaimDate advances to today the moment a user claims, this is exactly "claimed yesterday, not yet today" (and implies streak >= 1).
• Message is Postman-James-voiced and tailored to the streak length.

Supporting changes:

• _notifications.ts: export the existing sendToUser FCM helper for reuse.
• _dateUtils.ts: add previousDay and getLondonHourMinute helpers.
• New opt-out pref streakReminder (default on): defaulted in onUserCreated, gated with the existing === false convention, and surfaced as a Switch in the Settings → Notifications section (lib/settings_screen.dart).
• firestore.rules: explicit server-only block for notificationState (consistent with reportQuotas/trustScores).
• CLAUDE.md: documented the new function, helpers, and pref.

Tests

Added unit tests (functions/src/test/test.index.ts) for the slot maths (slotForLondonTime), the pure fire-decision (decideFire — random roll, fires at/after target, never twice/day, drops stale sentDay), pref gating (shouldNotifyStreakReminder), the previousDay/getLondonHourMinute helpers, the recipient query (mock Firestore), and the fan-out send (opt-out + allSettled). The region-pinning loop auto-covers the new export (europe-west2). All 33 new/related assertions pass; npm run build and eslint are clean.

Heads-up (pre-existing, unrelated): master currently references functions/src/onUserDeleted.ts and functions/src/_accountDeletion.ts (from the GDPR account-deletion feature), but those two files are absent from the repo, so the functions project does not compile as-is. This PR does not touch that area; I verified this branch's build/tests against temporary local stubs for those two modules, which are not included in the diff. Worth fixing separately.

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https://claude.ai/code/session_01S9hCUG7puzA94Ed46oJASU

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