fix: restore diff editor interactions

[levivilet]

Fixes the diff editor regressions found during desktop testing:

• inherit the active editor theme background, foreground, and cursor colors
• restore input event wiring and cursor-relative typing
• clamp click positions to the selected line
• preserve syntax highlighting when one side is a data URI
• exercise real wheel events and re-enable focused interaction E2E coverage

Validation:

• npm test (318 tests)
• npm run type-check
• npm run lint
• focused Chromium E2E tests for typing and mouse-wheel scrolling
• broader Chromium E2E run covering the newly enabled syntax, light-theme, and cursor tests