Bring sliding sync out of experimental

[7w1]

Description

Up for further discussion.

Thus far I've done the following:

• Moved sliding sync into a shared "Data Sync" general section with the settings sync
• Added a sliding sync toggle to the login screen, added a "swap to sliding/classic sync" clickable message to the loading screen.
• Removed the instance host option that force disables sliding sync for an instance (afaik, it's usable ootb on Synapse, c10y, and tuwunel, so no reason to force it to be disabled). Sliding sync is still disabled by default, it's just more accessible now and should allow larger accounts to actually log in instead of using work arounds to log in so they can toggle the setting.
• UI shell is rendered prior to full sdk init, technically means things load ever so slightly faster, but also feels faster since it's less loading screen and more UI shell. Also applies to standard sync.
• Removed fallback loading screens from config and spec version checkers to prevent brief flickers of their messages and added the background grid to the base index.html to remove the flicker of white prior to any other files loading in.
• Made the initial config fetch & versions check async, probably only saving a few milliseconds but easy enough. Applies to standard sync as well.

And still plan to:

• Fix known bugs...

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

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