feat(monospace): add settings schema + safe defaults

getDefaults(), getActiveMonoFont(), isLigaturesEnabled() with TDD.

tests/monospace-settings.test.js

@@ -0,0 +1,22 @@
+const { getDefaults, getActiveMonoFont, isLigaturesEnabled } = require('../src/main/settings/monospaceSettings');
+
+describe('monospaceSettings', () => {
+  test('getDefaults returns sane defaults', () => {
+    const d = getDefaults();
+    expect(d.monospaceFont).toBe('jetbrains-mono');
+    expect(d.monospaceLigatures).toBe(false);
+  });
+
+  test('getActiveMonoFont returns the active family', () => {
+    expect(getActiveMonoFont({ monospaceFont: 'fira-code' })).toBe('Fira Code');
+    expect(getActiveMonoFont({})).toBe('JetBrains Mono');
+    expect(getActiveMonoFont({ monospaceFont: 'bogus' })).toBe('JetBrains Mono');
+  });
+
+  test('isLigaturesEnabled reads boolean strictly', () => {
+    expect(isLigaturesEnabled({ monospaceLigatures: true })).toBe(true);
+    expect(isLigaturesEnabled({ monospaceLigatures: false })).toBe(false);
+    expect(isLigaturesEnabled({})).toBe(false);
+    expect(isLigaturesEnabled({ monospaceLigatures: 'yes' })).toBe(false);
+  });
+});