fix(sidebar): match native icon size

[Ericwong5021]

Closes #16

Changes

• Read the native Codex sidebar SVG's computed width and height.
• Apply those dimensions to the injected Better Codex navigation icon through CSS custom properties.
• Keep the existing 16px design-token size as a compatibility fallback.
• Regenerate the injected UI bundle and add regression coverage.

Validation

• npm test — 288 tests, 275 passed, 13 skipped, 0 failed
• npm run typecheck — passed