chore: prepare release 1.20.0

[sable-actions]

Preview the next release on dev.sable.moe

Important

Merging this PR will create a new release.

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. (#1071 by @eleboucher)
• Show the unread mention count in the page title. (#1093 by @eleboucher)

Improve sliding sync. (#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. (#1086 by @7w1)
• Allow users to send messages in encrypted rooms when they have permission to send m.room.encrypted events. (#1098 by @7w1)
• Allow users without a locally configured LiveKit focus to join calls started by others. (#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). (#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. (#1095 by @eleboucher)
• Fix redacted messages rendering as "Unsupported message (no body)" instead of "Message deleted" with sliding sync. (#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. (#1090 by @eleboucher)
• Fix joining spaces with sliding sync enabled. (#1085 by @eleboucher)
• Display the explicit room name for non-direct channels with two members. (#1098 by @7w1)
• Fix resetting animal identity (#1070 by @nushea)
• Fix the sliding-sync presence fallback consuming one-time-key updates and to-device messages without processing them. (#1078 by @7w1)
• Fix clickable handles (#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. (#1073 by @eleboucher)
• Add uploaded Sable CSS theme loading, automatic catalog updates, and a cleaner theme browser with full CSS viewing. (#1087 by @7w1)
• Show room members in developer tools. (#1088 by @7w1)