Deliver native updates and finish workspace regression fixes

[SergeiNikolenko]

Result

Prepare Burette 2.3.21 with native Sparkle updates and the workspace fixes from PR #714. Existing 2.3.18/2.3.19 clients retain the ZIP/checksum migration route; subsequent releases use signed native update dialogs and background downloads.

Changes

• Preserve unsaved documents across update cancellation, allow the native update command, and display downloaded-update status correctly.
• Package pinned Sparkle, configure signing and recoverable appcasts, and support in-app updates for Homebrew installations.
• Include single-molecule CPU generation with analytic MMFF gradients, renderer view continuity, file-tab menus and drag routing, Story presentation, and default-handler prompt fixes from PR Fix interactive generation and preserve molecular workspace views #714.
• Fix settings switches: independent CSS translate no longer doubles the thumb movement.
• Align strict Clippy and storage contracts with the merged implementation; document this Mac as the native build/acceptance host.

Validation

Actual Sparkle Install and Relaunch from another window detected unsaved source changes. Cancel sent an AppKit negative reply and preserved the visible buffer. After saving, retry installed the signed synthetic update and relaunched it; the new process loaded the molecule without errors. Archive signatures and the unchanged 2.3.18 bundle validator passed.

Settings switch browser verification passed in both themes and both states: 36×20 track, 16×16 thumb, 2-pixel edge inset. Merged UI suite passed. Combined macOS release build passed on this Mac. Actual Generate 3D on the regression molecule succeeded in 515 ms and opened the resulting conformer in the native viewer. Feed publication failure/retry tests pass under both system Bash 3.2 and Homebrew Bash. Public migration is the post-publication acceptance step.

A repeated local pre-push run stalled in the Bun HTTP preview test after the previous full pass. The same test passed under system Node; the final change only affects feed publishing and passed its focused suite under both Bash versions. GitHub validation is running for the final commit.