Lataa Mailspring 1.21.1

Performance:
- Optimized search regex compilation in category and label pickers. (#2715)
- Improved sync engine performance by reducing unnecessary SQLite writes and moving parsing outside transactions.
Bug Fixes:
- Improved theme support in calendar
- Fixed dark mode email bodies not reflecting the system tint color.
- Fixed activeMarks access errors in the composer editor. (#2714)
- Fixed thread list scroll indicator not sliding through time when dragging the scroll handle. (#2716)
- Fixed contains error when parentFilter receives a non-Element node. (#2712)
- Fixed secrets potentially appearing in the connection error detail modal. (#2717)
- Fixed package name validation rejecting names with uppercase letters. (#2718)
- Fixed sync engine crash during many-account sync by increasing SQLite busy timeout.
- Fixed message importance headers not normalized for compatibility with some providers. (mailsync#113)
- Improved error logging for empty or malformed error objects. (#2713)
Developer:
- Upgraded Electron from 41.5.0 to 41.6.1.
- Upgraded React and React DOM to v17. (#2725)
- Upgraded react-transition-group from 1.2 to 4.4.5.
- Replaced deprecated componentWillMount with componentDidMount. (#2724)
- Replaced legacy context API with React Context. (#2722, #2723)
- Refactored class components to functional components with React.memo. (#2720)
- Removed PropTypes, replaced with TypeScript types. (#2710)
- Added missing TypeScript types to function declarations across many files.
- Renamed remaining package spec directories to specs, converted JSX to TSX. (#2711)
- Added Playwright tests for composer interactions.
- Fixed lint CI step that was performing fixes instead of validation. (#2719)
- Removed unused CircleCI configuration.
- Set C++ standard to C++17 across all platforms. (mailsync#114)
- Various CMake and build improvements for mailsync. (mailsync#108, mailsync#109, mailsync#110, mailsync#111, mailsync#112) Thanks @LinusDierheimer!