fix: classify SDG bingo missions correctly instead of daily_challenge (#221)

[vedant7007]

Summary

`Bingo.tsx` dispatched every mission tick as a bare `ADD_POINTS` with only a numeric payload. GameContext's reducer at line 589-591 falls back to `'daily_challenge'` when no `activityType` is provided, so all 51 SDG bingo tasks landed in `xp_ledger` with `activity_type = 'daily_challenge'`.

That corrupts:

• the daily-challenge counter used for stats/streaks (bingo activity inflates it)
• any per-day cap on daily_challenge (bingo eats into it and starves real daily challenges — or vice versa)
• badge counts that filter `xp_ledger` by activity type

What changed

• added `bingo_mission` to the `ActivityType` union in `src/lib/gamification.ts`
• `Bingo.tsx:373` now dispatches with `activityType: 'bingo_mission'` and `metadata: { goalIndex, taskIndex, sdgTitle }` so the ledger row is queryable and consistent with the pattern used everywhere else in the codebase (`event_participation`, `community_post`, etc.)

Follow-up needed on the backend

The `award_xp_secure` RPC has an activity table on its side. If `bingo_mission` isn't in it, the RPC will either fall back to a default XP calc or reject the activity. Happy to iterate on that in a follow-up once the maintainer flags which behavior applies — the frontend change here is the necessary first step regardless.

Test plan

• typecheck clean on both files
• verified the fallback in GameContext:589-591 is the sole cause of misclassification
• once approved: solve 3 bingo missions, verify 3 new `xp_ledger` rows appear with `activity_type = 'bingo_mission'`, not `'daily_challenge'`

Fixes #221

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[vedant7007]

hey! heads up — the failing Verify Codebase check is red because of a pre-existing issue on master: src/App.tsx:13 imports ./pages/Auth but no Auth file exists in src/pages/. every PR opened right now fails on the same import resolution error (see [Failed to resolve import "./pages/Auth" from "src/App.tsx"] in the run log).

this PR doesn't touch App.tsx or anything in src/pages/, so nothing here is causing it. happy to open a separate PR restoring the Auth page (or wiring the route to whatever the intended replacement is) if you can point me at what should live there. thanks!