Refresh CodeSense interface with flat teal design

[XiaoCow666]

Changes

• Replace first-party gradients and blue-purple surfaces with a flat teal palette.
• Redesign the code studio assistant, evidence panel, and login view for desktop and narrow screens.
• Preserve existing Jinja and JavaScript interfaces while improving responsive layout.

Verification

• 25 targeted tests passed.
• Browser checks at 1440, 768, 390, and 320 px; no horizontal overflow.