fix: resolve icon inheritance and synchronize tooltip UI with GitHub native style

[ImXiangYu]

This PR addresses several visual inconsistencies and a bug where custom navigation buttons incorrectly inherited icons from their template sources. It also synchronizes the hotkey tooltip appearance with GitHub's native UI.

Key Changes

• Fix Icon Inheritance: Updated src/navigation.js to explicitly clear existing <svg> icons when setting link text. This prevents custom buttons from incorrectly showing "fork" icons when cloned from repository breadcrumbs.
• UI Synchronization: Refined tooltip and hotkey chord (shortcut) styles in src/styles.js to match GitHub's native Primer design:
  • Adaptive Colors: Switched to native CSS variables for better theme compatibility.
  • Refined Chords: Adjusted chord dimensions, font weight (400), and spacing to look more compact and professional.
  • Bug Fix: Explicitly disabled text-decoration and border on tooltip elements to eliminate unwanted connecting lines between keys.
  • Tactile Feel: Added subtle box-shadows to chords to simulate the native 3D button effect.
• Build & Versioning: Bumped version from 0.1.33 to 0.1.38 and updated the compiled better-github-nav.user.js.

Verification

• Confirmed custom buttons no longer display incorrect cloned icons.
• Verified that the hotkey tooltip style is consistent with GitHub's native shortcut panel across different themes.