frontend: redo sign-in step as provider table with dropdown

[MekDrop]

Summary

• Replaces the radio-button provider selector with a Win98-styled two-row table (GitHub / GitLab)
• Adds a Sign In ▾ dropdown button in the top-right toolbar to initiate either provider's OAuth/PKCE flow
• Each table row shows the current auth status ("Signed in as …" / "Not signed in") and a per-row sign-in or sign-out link
• The Next > wizard button remains disabled until at least one provider is authenticated
• GitLab host input still appears (below the table) when a GitLab sign-in is pending or active

Closes #49

Test plan

• Visit /login — table renders with GitHub and GitLab rows, both showing "Not signed in"
• Click Sign In ▾ — dropdown opens with two items; clicking outside closes it
• Sign in with GitHub — row updates to "Signed in as <user>", Next button becomes enabled
• Sign out via row link — row reverts to "Not signed in", Next disabled again
• Sign in with GitLab — host input appears below table; completing OAuth updates the row
• Already-signed-in provider item in dropdown is disabled