fix open renderer defects

[cat0825]

Summary

• Fixes Editor serializer inserts trailing backslash hard-breaks and strips blank lines on save #798 by stripping BlockNote single trailing soft-break artifacts from compact markdown while preserving authored hard breaks and code blocks.
• Fixes macOS: mouse cursor selects in the open dialog/palette if it happens to be in the wrong spot #819 by ignoring stationary mousemove events in palette/search hover selection until the pointer actually moves.
• Fixes Right-click menu display issue #855 by sharing viewport-aware note-list context menu positioning across normal and changes menus, with max-height scrolling.
• Fixes When typing in Chinese, the first keystroke disappears. #850 and Korean IME input loses previously typed characters during composition #851 by centralizing IME composition guards for app shortcuts and rich editor input transforms.

Root Cause

• BlockNote serialization emitted single trailing backslashes that looked like authored markdown.
• Palette rows treated synthetic/stationary mousemove events as intent, stealing keyboard selection.
• Note-list context menus used raw click positions without clamping/flipping against viewport bounds.
• Composition keydown/beforeinput events could flow through shortcut and transform handlers during Windows Chinese/Korean IME input.

Validation

• pnpm exec vitest run src/utils/compact-markdown.test.ts src/utils/mouseMovement.test.ts src/utils/imeEvents.test.ts src/components/note-list/contextMenuPosition.test.ts src/components/richEditorInputTransformExtension.test.ts src/components/imeCompositionKeyGuardExtension.test.ts src/hooks/useAppKeyboard.test.ts src/components/CommandPalette.test.tsx src/components/QuickOpenPalette.test.tsx src/components/NoteSearchList.test.tsx src/components/NoteList.contextMenu.test.tsx src/components/NoteList.changes.test.tsx
• npx tsc --noEmit
• pnpm lint
• Pre-commit hook: full Vitest 405 files / 4447 tests passed; lint and typecheck passed; CodeScene skipped because CODESCENE_PAT/CODESCENE_PROJECT_ID were not set.
• Pre-push hook: lint passed; TypeScript + Vite build passed; frontend coverage passed; Rust lint passed; Rust coverage passed; Playwright smoke 56/56 passed; CodeScene skipped because CODESCENE_PAT/CODESCENE_PROJECT_ID were not set.

Manual QA Not Run

• Windows 11 + Sogou Chinese IME and Windows 11 + Korean IME were not available in this environment.