feat: Multi-user account switching

[Pecacheu]

Split off from and baseline for multi-instance switching in:

• feat: Multi-account switching & self-hosted/on-premise instances #999

Requires:

• feat: Instance context to enable routes and reactivity w/ consistent interface #1315 (Mainly it has been rebased on this just to avoid merge conflict)

How was this PR tested?

At least a dozen furries daily-driving PR 999 for months :3

This has been briefly tested to ensure that extraction from the above didn't break it.

Screenshots & Screencasts (if appropriate)

Back button appears on auth page only when adding an additional account, not on first login. Matches styling of the light/dark mode button.

Checklist:

• I have carefully read the contributing guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if applicable
• I have no unrelated changes in the PR
• I have confirmed that any new dependencies are strictly necessary
• I have written tests for new code (if applicable)
• I have followed naming conventions/patterns in the surrounding code

Please declare, if any, LLM usage involved in creating this PR

80% Raichu powered, 20% V8 Energy drink at 2 AM powered

[Dadadah]

Not bad, I would appreciate if you could go in to detail about how the state store works now, perhaps we should update the developer documentation?

This PR is large, are you sure there aren't any more relevant screenshots or videos for ui/ux review?

[Pecacheu]

This PR is large, are you sure there aren't any more relevant screenshots or videos for ui/ux review?

Just added one more screenshot- But aside from that, this PR is more backend changes than UI so that's that's changed UI-wise. Eg. several of the changed JSX files was just to pass the onClose listener to the add account button so that it closes the settings UI when pressed.

Not bad, I would appreciate if you could go in to detail about how the state store works now, perhaps we should update the developer documentation?

Sure, updated documentation for the State/Store changes might make sense. Long story short, now instead of having only one database (currently named "localforage" in main, as that's what the library defaults to), we treat the old one as the 'global' DB that holds any store with the global flag set to true- namely the auth store at the moment. All other stores are instead loaded from and written to the appropriate store with the name instanceHost@userId - Since only default instance is supported at the moment, the former is blank, i.e. it is just @userId.

For forward compatibility, I made the multi-instance PR behave like this too, so that none of your settings for existing main-instance logins are lost in the void. In other words, he default instance (as set by CONFIGURATION) is always replaced with a blank string (even if you explicitly type in stoat.chat in the instance field) so that there is no instance prefix. And your first logged-in account stores everything in "localforage" instead, again so that existing settings are not lost.

One thing I didn't yet implement that should probably be added is garbage collection for unused DBs, i.e. after logout of a secondary account the data just sits there. The databases known to localforage should be double-checked against the list of saved sessions in the global auth store.