Gamification: Dashboard summary card + module-completion toast

[ljm20088]

Parent PRD

#41

What to build

Surface gamification stats in two remaining UI locations (merged slice):

1. Dashboard summary card (app/routes/dashboard.tsx) — a card showing current XP, level, progress toward next level, current streak, and longest streak, shown when the student logs in. See PRD "UI Placement" (Dashboard), user story 11.
2. Module-completion toast — when a student completes the last lesson in a module, show a Sonner toast with the total XP earned across that module's lessons, e.g. "Module complete! +40 XP earned". No per-lesson XP toast. See PRD "UI Placement" (Module completion toast), user story 10.

Both consume stats/XP already produced by the foundation and streak slices.

Acceptance criteria

• Dashboard shows a summary card with XP, level, progress-to-next, current streak, and longest streak
• Completing the last lesson in a module triggers a toast showing total XP earned across that module
• Completing a non-final lesson shows no module toast
• Last-lesson-in-module detection uses lesson position ordering
• pnpm test and typecheck pass

Blocked by

• Blocked by Gamification: XP foundation — earn XP on lesson completion + level in sidebar #77
• Blocked by Gamification: Streaks — record activity + current/longest in sidebar #80

User stories addressed

• User story 10
• User story 11