refactor(tui): centralize UI colors in Theme tokens

[samzong]

What's changed?

• Add src/tui/theme.rs with a THEME token map for TUI colors
• Route search, popups, usage, viewing, and shared UI styles through theme tokens instead of ad-hoc Color::* literals
• Update existing UI style assertions to assert against THEME values

Why

• Color choices were scattered across render modules, which made palette changes error-prone and inconsistent
• A single token source keeps borders, selection, accents, roles, and usage series aligned without changing the current look

Verification

• cargo test -q --lib tui::ui — 9 passed
• cargo clippy -q --lib -- -D warnings — clean
• pre-ship --committed round 1/2 — MUST-FIX: 0