fix: double editor column min-width from 400px to 800px

Rework: increase editor minimum width per feedback. Updates CSS,
layout constants, tests, and tauri.conf.json minWidth (800→1200).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/hooks/useLayoutPanels.test.ts

@@ -6,7 +6,7 @@ describe('useLayoutPanels', () => {
   it('exports column minimum widths', () => {
     expect(COLUMN_MIN_WIDTHS.sidebar).toBe(180)
     expect(COLUMN_MIN_WIDTHS.noteList).toBe(220)
-    expect(COLUMN_MIN_WIDTHS.editor).toBe(400)
+    expect(COLUMN_MIN_WIDTHS.editor).toBe(800)
     expect(COLUMN_MIN_WIDTHS.inspector).toBe(240)
   })