fix: replace alert() with toast.error() for avatar upload validation (Closes #879) [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Summary

Replaces the native alert() calls in ProfileTab.handleFileUpload with toast.error() from the sonner toast system, matching the pattern used everywhere else in this file. Also resets e.target.value after each validation failure so the same file can be re-selected.

Changes

• ProfileTab.tsx: Replace both alert(...) calls with toast.error(...) using the same messages
• ProfileTab.tsx: Reset e.target.value = '' after each validation failure so re-selecting the same file re-triggers onChange
• ProfileTab.test.tsx: Add 3 tests covering:
  • Invalid file type triggers toast.error with the correct message
  • Oversized file triggers toast.error with the correct message
  • Re-selecting the same rejected file re-runs validation (input value reset)

Closes #879