[MekDrop]

Summary

• Add frontend/src/services/github.js with fetchCurrentUser(token) for GitHub API calls
• Add frontend/src/stores/auth.js — Pinia setup store with isLoggedIn getter, loginWithToken, and logout actions; hydrates from localStorage on creation
• Add frontend/src/pages/LoginPage.vue — GitHub OAuth redirect and GitLab PKCE login with host input and error banner support
• Add frontend/src/pages/AuthCallback.vue — handles both /auth/github and /auth/gitlab redirect callbacks, exchanges tokens and redirects to /repos
• Update frontend/src/router/routes.js to register the three new routes under MainLayout

Closes #8

Test plan

• Navigate to /#/login — GitHub and GitLab login buttons are visible
• Clicking "Login with GitHub" redirects to GitHub OAuth with correct client_id and redirect_uri
• Clicking "Login with GitLab" (with host filled in) starts the PKCE flow
• /#/login?error=no_token shows the error banner
• After a successful GitHub callback, auth state is persisted in localStorage and the user is redirected to /repos
• After a successful GitLab callback, same behaviour as above
• logout() clears state and removes the localStorage key
• Refreshing the page restores auth state from localStorage