Per-user saved jobs fixed + polished Jobs UI with styling and tests

[kassandrabeas5]

• Auth

  • Exported useAuth() from context/AuthProvider for easy access to username/token
  • Register now calls login(...) after creating a user; Login continues to call login(...)

• Jobs page UX/UI

  • Styled to team palette (white bg, #00A7E1 primary, #00171F text, etc.)
  • Kept query chip showing the active category

• Saved Jobs page UI

  • Styled cards, improved spacing, added clear “View Posting” and “Remove” actions

• Tests

  • Updated jobs.test to match new UI (chip, heart toggle via testID)
  • Added savedJobs.test to verify hydrate/add/remove/isSaved with user-scoped storage
  • Added adzuna.test (happy path + error path with mocked fetch)
  • Test setup: mock Linking + AsyncStorage to avoid native warnings