feat(editor): add typewriter mode

[murongg]

Summary

• add a persisted Typewriter Mode toggle to editor settings, including all supported translations
• keep the focused caret line centered in visual and source editors across main, split, and side panes
• add dynamic edge gutters plus focus, mirrored-update, resize, and IME composition safeguards
• cover preference persistence and editor scrolling behavior with focused regression tests

Why

Typewriter Mode keeps the active writing line near the center of the current editor pane, making long-form editing easier to track on larger displays.

Validation

• pnpm test passed
• pnpm typecheck:test passed
• pnpm build passed
• git diff --check origin/main...HEAD passed

Risk

• the feature is disabled by default and can be turned off from Editor settings
• automatic scrolling is limited to the focused pane and pauses during IME composition
• manual desktop recording was not run; the interaction is covered by automated visual/source/split-pane tests

Screenshots

Not included because the main change is scrolling behavior rather than a static visual state.

Closes #536

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Project	Deployment	Actions	Updated (UTC)
markra-web	Ready	Preview, Comment	Jul 18, 2026 1:17am