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 sync controls for supported homeservers. (#1117 by @7w1)
• Add desktop keyboard shortcuts to open settings, toggle window visibility, and quit. (#1153 by @7w1, @eleboucher)
• Improve desktop window layout with seamless macOS titlebars and custom Linux controls. (#1153 by @7w1, @eleboucher)
• Improve mobile UX with native back button support, haptics, and iOS keyboard handling. (#1153 by @7w1, @eleboucher)
• Share content via native system share menu or copy links on desktop. (#1148 by @eleboucher)
• Share text, media, and files into Sable directly from other mobile apps. (#1159 by @7w1, @eleboucher, @mortebrume)
• Add beta builds for desktop and mobile. (#1103 by @7w1, @eleboucher, @hazre, @mortebrume, @nikiwastaken, @TastelessVoid)

Fixes

• Added a modal for viewing room details on mobile. (#1173 by @7w1)
• Add account switching and logout actions to the offline screen. (#1115 by @7w1)
• Improve Android media uploads with streaming progress tracking and automatic temp file cleanup. (#1154 by @eleboucher)
• Cache room metadata for faster sliding sync warm starts. (#1118 by @7w1)
• Fix media loading stalling on Android when browsing history. (#1129 by @eleboucher)
• Keep message input focused after selecting autocomplete suggestions on mobile. (#1103 by @7w1, @eleboucher, @hazre, @mortebrume, @nikiwastaken, @TastelessVoid)
• Fix missing profile pictures for bridged users in timeline when using sliding sync. (#1112 by @7w1, @eleboucher)
• Fix room leaving state not updating immediately in UI. (#1146 by @7w1)
• Smooth out animation when tapping 'following the conversation'. (#1173 by @7w1)
• Fix keyboard flickering when editing messages on mobile. (#1130 by @eleboucher)
• Improve mobile touch feedback and prevent accidental presses. (#1173 by @7w1)
• Smoother mobile navigation and faster room opening. (#1144 by @eleboucher)
• Fix missing user display names under sliding sync across search, notifications, threads, and replies. (#1170 by @7w1)
• Fix display names in read receipts and event details when using sliding sync with member drawer closed. (#1111 by @7w1)
• Fix sliding sync not subscribing to call updates. (#1173 by @7w1)
• Fix subspaces with emote packs listed as separate spaces under sliding sync. (#1119 by @mortebrume)
• Fix space icons staying blank on first open with sliding sync. (#1112 by @7w1, @eleboucher)
• Fix loading space-hosted sticker and emote packs when using sliding sync. (#1112 by @7w1, @eleboucher)
• Fix timeline auto-scrolling to new messages when pinned to the bottom. (#1105 by @eleboucher)
• Prevent typing indicator from obscuring messages and stabilize timeline scroll snapping. (#1113 by @eleboucher)
• Various visual tweaks (#1114 by @nushea)
• Fix overlapping Bookmarks and Bold Text shortcut (#1120 by @cyrneko)
• Improve app startup speed and eliminate initial launch screen flickering. (#1127 by @7w1, @eleboucher)
• Polish the account panel and settings layout on mobile. (#1131 by @eleboucher)
• Improve message rendering consistency across views. (#1170 by @7w1)
• Various performance improvements. (#1184 by @7w1)