Rework light theme readability and contrast #79

Summary

The Raven TUI light theme needs a visual pass. In the current light theme, the start screen looks washed out and several UI elements have poor contrast against a white terminal background.

Observed issues

• The large RAVEN AGENT banner is too pale on a light background and becomes hard to read.
• The raven logo / ASCII art uses low-contrast olive tones that blur into the page.
• Headings, metadata, and separators do not have a strong enough hierarchy in light mode.
• The bordered welcome panel feels visually heavy while the content inside feels faint.
• The light theme needs to be checked across the TUI, not only the welcome screen.

Suggested scope

• Define a dedicated light-theme palette instead of relying on dark-theme colors on a light background.
• Improve contrast for banner text, ASCII art, headings, muted text, borders, command hints, and status metadata.
• Review key TUI surfaces: welcome screen, chat transcript, input area, tool/skill lists, status bars, dialogs, and error/warning states.
• Keep dark theme behavior unchanged.

Acceptance criteria

• Light theme text and important UI states are readable on a white or near-white terminal background.
• The welcome screen has clear visual hierarchy without looking washed out.
• Borders and separators are visible but not overpowering.
• The theme is verified manually in a light terminal profile, with before/after screenshots added to the PR.