feat(tui): tint the user's own turns in the transcript

[emal-avala]

Summary

User turns render with no background tint, so scanning back through a long transcript there is nothing distinguishing what you typed from what the model said. Peer terminal agents tint the user's own input; we didn't.

The cause: three dead theme slots

The theme already derives these from the active palette:

user_message_bg:   mix(p.blue,   p.bg, 0.88),
bash_message_bg:   mix(p.green,  p.bg, 0.90),
memory_message_bg: mix(p.purple, p.bg, 0.90),

…and nothing consumed any of them. The renderer painted user turns with a hardcoded Color::White foreground and no background at all — the same "built but never wired" pattern as the unused word-diff colours and the parsed-then-discarded hyperlinks.

What changed

• Apply the tint across the whole user line (marker + body + one trailing cell, so the highlight reads as a block rather than stopping flush against the last glyph).
• The prefix selects the tint, which is exactly what the three slots were designed for per their doc comments: a plain prompt, a ! shell passthrough, and a # memory note are different kinds of input and now read differently.
• The marker is stripped from the body instead of being duplicated (!ls -la previously rendered the ! twice).
• Foreground now comes from the theme instead of a hardcoded colour, so the line participates in colour degradation on limited-colour terminals like everything else.

Verification

• Three unit tests: every span after the gutter is tinted; the three input kinds get distinct tints; the marker isn't duplicated into the body.
• Driven live in the TUI and verified at the escape-sequence level — the emitted line now carries 48;5;… (background) across the marker and text with a themed 38;5;… foreground, where before it had no background and a hardcoded white.
• cargo test, clippy --all-targets -- -D warnings, fmt --check all clean.

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