Refactor UI navigation and improve visual consistency with SVGs

[normal-coder]

Summary

Adjustments have been made to the UI details of the OpenFang Web Console to improve overall UI/UX consistency and product experience:

1. Optimized sidebar navigation and adjusted UI element sizes

   • Overview is now the first navigation item, which is more in line with the existing modern Web Console usage habits (Chat remains the default entry)
   • The Chat window now has both the function to create a new agent and to start a new conversation, which is more suitable for categorization under the "Agent" section
   • Optimized primary navigation: diamond icon size (8px → 16px), increased font size of some titles (9px → 12px) to improve visibility

2. To address the visual weakness of text color under the "Dark" mode, CSS theme markers have been introduced to achieve consistent emphasis text contrast

   • Added the CSS variable --text-on-accent (white) for emphasizing text on the background
   • Replaced the hardcoded var(--bg-primary) in components with --text-on-accent
   • Updated the surface, border, and text color markers of the dark theme to improve contrast
   • Adjusted the surface color of the dark mode (#1F1D1C → #242221) to enhance the sense of hierarchy
   • Optimized border and text fade colors to enhance visual hierarchy

3. Inline SVG has been used to replace HTML entity icons to improve visual consistency

   • In all modal windows and panels, the × close icon has been replaced with an SVG X icon
   • In the settings list, step indicators, and success status, the ✓ checkmark has been replaced with an SVG check icon
   • The • and · separators have been replaced with small SVG dot icons to improve visual consistency
   • Improved icon size, alignment, and stroke properties for clearer rendering

4. Fixed the issue where the browser refused to load the Logo due to inconsistent definition of icon size between logo.png and manifest.json

Changes

1. Optimized sidebar navigation and adjusted UI element sizes
2. To address the visual weakness of text color under the "Dark" mode, CSS theme markers have been introduced to achieve consistent emphasis text contrast
   
3. Inline SVG has been used to replace HTML entity icons to improve visual consistency
   

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable)

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries