TUI: inherit terminal background colors

[wesm]

Summary

• Remove forced #ffffff/#000000 backgrounds from TUI styles so the interface respects the user's terminal color scheme
• Cursor row, title bar, alt rows, and search highlight retain explicit backgrounds for contrast

Addresses user feedback that the TUI clashes with non-default terminal themes.

🤖 Generated with Claude Code

[roborev-ci]

roborev: Combined Review (6d8ea06)

Verdict: Changes are mostly sound, but there is one medium-severity rendering regression to address before merge.

Medium

• internal/tui/view.go:69, internal/tui/view.go:1422, internal/tui/view.go:1457
  Removing the modal background introduces a rendering regression in overlayModal(). The modal is assembled by splicing ANSI-styled background fragments with truncateToWidth() and skipToWidth(), so if the left fragment ends while a background color is still active (for example from cursorRowStyle or altRowStyle), the unstyled modal content can inherit that background. This causes the modal body and padding to render inconsistently instead of using the terminal default background.
  Suggested fix: keep an explicit background on modalStyle, or explicitly reset/apply the intended background before writing each modalLine into the composed output.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)

[roborev-ci]

roborev: Combined Review (25b684c)

Verdict: No medium-or-higher issues found.

All reviewed agents agree this PR is clean at Medium, High, and Critical severity.

Briefly: the diff is limited to TUI background/styling behavior in internal/tui/view.go, with explicit backgrounds retained where needed for contrast and modal compositing, and no security-relevant regressions were identified.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)