feat: wire sidebar, toast notifications, and fix module completion state

[DeFiVC]

Summary

• Import Sidebar component into dashboard layout (hidden on mobile, visible on lg+ screens)
• Add context-based ToastProvider for shared cross-component toast state
• Add toast notifications on wallet connect, enrollment, module completion, and reward claiming
• Fix ModuleList always showing modules as incomplete — now passes enrollment.completedModules instead of []

Closes #9, #10, #11

Test plan

1. Open app on desktop (>1024px) — sidebar visible with nav links
2. Resize to mobile — sidebar hidden, MobileNav appears
3. Connect wallet → success toast appears
4. Enroll in course → success toast appears
5. Complete a module → success toast appears
6. Return to course detail → completed modules show green checkmarks
7. Claim a reward → success toast appears

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[DeFiVC]

Review Summary

Retargeted to main and rebased — no more merge conflicts.

What's Good

• ToastContextProvider uses createContext + useContext for shared state
• useCallback on addToast/removeToast prevents unnecessary re-renders
• Module completion fix: enrollment?.completedModules ?? [] from Zustand store
• All pages converted from local useToast to shared useToastContext

Note

• Sidebar wiring already in main (PRs feat(#2): integrate Sidebar into dashboard layout #5/feat(#3): integrate Toast notifications into auth, enrollment, and reward flows #6) — this PR only adds toast context + module fix