Bring sliding sync out of experimental - #1069

> **Preview the next release on [dev.sable.moe](https://dev.sable.moe)**

> [!IMPORTANT]
> Merging this PR will create a new release.

## Security

* This release includes an important security fix. Please update as soon
as possible. Sensitive Matrix account data is not exposed by this issue.
([SableClient#1107](<SableClient#1107>) by @7w1)

## Features

* Add Native OIDC (OAuth 2.0) login so you can sign in to and register
on homeservers that use delegated authentication, such as continuwuity
and Synapse with the Matrix Authentication Service. Sessions stay signed
in through automatic token refresh, and signing out revokes the tokens
on the server.
([SableClient#1071](<SableClient#1071>) by
@eleboucher)
* Show the unread mention count in the page title.
([SableClient#1093](<SableClient#1093>) by
@eleboucher)

### Improve sliding sync.
([SableClient#1069](<SableClient#1069>) by @7w1,
@eleboucher)

- Make sliding sync easier to enable during login or loading and from
settings.
- Move the sliding sync toggle from Experimental to the bottom of
General settings.
- Start faster and restore rooms and spaces sooner.
- Reduce unnecessary background room data.
- Improve room details, memberships, presence, and unread indicators.
- Make startup loading smoother for both sliding and classic sync.
- It's fast now :3

## Fixes

* Use attachment filenames instead of captions when downloading files
and harden unsafe download names.
([SableClient#1086](<SableClient#1086>) by @7w1)
* Allow users to send messages in encrypted rooms when they have
permission to send `m.room.encrypted` events.
([SableClient#1098](<SableClient#1098>) by @7w1)
* Allow users without a locally configured LiveKit focus to join calls
started by others.
([SableClient#1099](<SableClient#1099>) by @7w1)
* Fix the new messages divider not appearing when the read receipt
points at an event hidden from the timeline (reaction, edit, thread
reply). ([SableClient#1101](<SableClient#1101>) by
@eleboucher)
* Fix the new messages divider staying at the room-open position instead
of re-anchoring at the last read when the window loses focus.
([SableClient#1095](<SableClient#1095>) by
@eleboucher)
* Fix redacted messages rendering as "Unsupported message (no body)"
instead of "Message deleted" with sliding sync.
([SableClient#1089](<SableClient#1089>) by
@eleboucher)
* Display redacted state events (membership, room name/topic/avatar,
pins and other state events) as a "deleted" tombstone in the timeline
instead of rendering them with empty/redacted content or hiding them, so
the reply-to of a redaction highlights the redacted event.
([SableClient#1090](<SableClient#1090>) by
@eleboucher)
* Fix joining spaces with sliding sync enabled.
([SableClient#1085](<SableClient#1085>) by
@eleboucher)
* Display the explicit room name for non-direct channels with two
members. ([SableClient#1098](<SableClient#1098>) by
@7w1)
* Fix resetting animal identity
([SableClient#1070](<SableClient#1070>) by @nushea)
* Fix the sliding-sync presence fallback consuming one-time-key updates
and to-device messages without processing them.
([SableClient#1078](<SableClient#1078>) by @7w1)
* Fix clickable handles
([SableClient#1068](<SableClient#1068>) by @nushea)
* Hide the "Register" link on the login page when the homeserver has
registration disabled and offers no SSO to register through, so you are
no longer sent to a dead-end registration page.
([SableClient#1073](<SableClient#1073>) by
@eleboucher)
* Add uploaded Sable CSS theme loading, automatic catalog updates, and a
cleaner theme browser with full CSS viewing.
([SableClient#1087](<SableClient#1087>) by @7w1)
* Show room members in developer tools.
([SableClient#1088](<SableClient#1088>) by @7w1)