Clean up login page styles

[toolmantim]

• Moves the Enterprise sign in link up under the social login links
• Makes the button logos monochrome
• Updates the window sizing and button width behaviour to match standard VS Code behaviour

Before	After

Test plan

• Loaded extension
• Previewed widths and clicked buttons

[toolmantim]

@dominiccooney I also tried fixing the "Sourcegraph.com" that erroneously appears in the "Recent" list when you click "Sign In To Enterprise Instance" but failed (I think perhaps its coming from https://github.com/sourcegraph/cody/blob/main/vscode/src/services/AuthProvider.ts#L377)

[dominiccooney]

Monochrome sign in logos look great!

[dominiccooney]

I also tried fixing the "Sourcegraph.com" that erroneously appears in the "Recent" list when you click "Sign In To Enterprise Instance"

I wrote that FIXME and it is apt, the code is racy. We should add a state machine here so we don't mix state from one sign in to another. Once we remove App it gets a bit simpler and we can lean into that to fix these.