Introduce a better and more compact login action button

Problem to solve

Currently, the login flow includes selecting on of the three providers (GitLab, GitHub, Bitbucket) on the login page. However, buttons are using a secondary weight to avoid having multiple primary buttons, does not allow flexible solutions in cases like https://github.com/gitpod-io/gitpod/issues/6826, etc.

Proposal

Introduce a dropdown button for selecting one of the available providers with a single-click action to allow having a single prmary action button with proper button weight.

Benefits among others include:

1. Better user experience with a clear and single action button.
2. Allowing better and more flexible UX for cases like https://github.com/gitpod-io/gitpod/issues/6826.
3. Support more providers without making the interface crowded. For example, see https://github.com/gitpod-io/gitpod/issues/80.

PROPOSAL

BEFORE	AFTER	AFTER (Hover)	AFTER (Active)

See design specs. 🖍️

Start with the linked Figma design specs and locate the login page implementation that renders the GitLab, GitHub, and Bitbucket provider buttons. Compare the current flow with the proposed dropdown states and confirm how provider selection triggers login. Done means the login page has one compact primary action, supports all listed providers, and matches the documented hover and active designs.