[feature/frontend] Add login button to index page which reiterates info about clients

[vivlim]

Description

This PR adds a 'Log in' button to the index page. When clicked, a page will be shown which contains

• A link to access /settings
• The list of client applications that is also present on the index

The goal is to provide an easier way to discover how to log on / access the settings without having to read and understand all of the text on the index first. I was talking with someone recently who had set up an account but wasn't sure what to do next; they had skimmed the index page but their eyes glossed over the part where there was a list of software with different logos. I think reiterating just that information when a user is trying to log in is an improvement.

I also think it's nice to have a way to get to settings that doesn't require typing /settings in the address bar.

This change is live at https://electron.phosphor.buzz if you want to try it out. I tried to make it look OK at different widths and think it turned out alright.

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.