chore: prepare release 1.21.0

[sable-actions]

Preview the next release on dev.sable.moe

Important

Merging this PR will create a new release.

Features

• Add MSC4466 profile change propagation controls for homeservers that advertise support. (#1117 by @7w1)
• Add beta builds for desktop and mobile. (#1103 by @7w1, @eleboucher, @hazre, @mortebrume, @nikiwastaken, @TastelessVoid)

Fixes

• Add account switching and logout actions to the homeserver offline screen. (#1115 by @7w1)
• Cache room banners, abbreviations, power-level labels, and widgets for faster sliding sync warm starts. (#1118 by @7w1)
• Fix media loading stalling on Android after browsing through message history. (#1129 by @eleboucher)
• Keep the message composer focused when tapping an autocomplete suggestion on mobile, so selecting an emoji, mention, or command no longer unfocuses the input. (#1103 by @7w1, @eleboucher, @hazre, @mortebrume, @nikiwastaken, @TastelessVoid)
• Fix missing profile pictures for bridged users in the timeline when using sliding sync. (#1112 by @7w1, @eleboucher)
• Fix editing messages on mobile where tapping the edit field flickered the keyboard shut. (#1130 by @eleboucher)
• Smoother mobile navigation and faster room opening. (#1144 by @eleboucher)
• Fix display names for read receipts, event relations, and state events when using sliding sync with the member drawer closed. (#1111 by @7w1)
• Fix a regression making subspaces with emote packs being listed as separate spaces on sliding-sync. (#1119 by @mortebrume)
• Fix space icons staying blank on first open with sliding sync until the sidebar was remounted. (#1112 by @7w1, @eleboucher)
• Fix sticker and emote packs hosted on space rooms not loading with sliding sync, and load pack rooms before the emoji board is first opened. (#1112 by @7w1, @eleboucher)
• Fix OIDC device removal, sessions dashboard, and cross-signing reset not opening a browser on Tauri mobile. (#1136 by @eleboucher)
• Fix the timeline not reliably following new messages when pinned to the bottom, and stop the browser's scroll anchoring from fighting the virtualized message list. (#1105 by @eleboucher)
• Show the typing indicator as its own row instead of an overlay so it no longer hides the last message, and stop the timeline from snapping to the bottom when you scroll back down after messages arrived off-screen. (#1113 by @eleboucher)
• Various visual tweaks (#1114 by @nushea)
• Fix overlapping Bookmarks and Bold Text shortcut (#1120 by @cyrneko)
• Speed up cold start by keeping the PDF viewer out of the initial bundle (it now loads on demand), cutting the amount of JavaScript fetched and parsed on launch. The desktop and mobile windows also paint the app's background color immediately, so startup no longer flashes a blank black screen before the first frame. (#1127 by @7w1, @eleboucher)
• Polish the account panel and settings layout on mobile. (#1131 by @eleboucher)