fix(tui): replace violet accent with banner green

[echobt]

Summary

• Replace violet with the reference banner's exact dominant green #1F4945.
• Keep focused green text readable on a contrasting #F5F5F5 backing. Preserve semantic success/diff colors.
• Update theme documentation and source renderers; existing exported images are historical, not regenerated.

Test plan

• Core style tests: 12 passed, including contrast assertion.
• TUI unit/headless tests: 1044 passed, 7 existing ignored.
• Components/capture tests: 93 and 46 passed.
• cargo fmt --all -- --check; git diff --check; Python renderer syntax.
• Full workspace, audit and CI not run/waited for this palette-only change, as requested.

Attestation

• Security: visual-only change, no auth/exec/path/network behavior changes.
• Product-facing errors: unchanged.
• TUI: unit and narrow/wide headless render assertions passed; live authenticated session not exercised.
• Tests added/updated with actual color and contrast assertions.
• No secrets included.

Risk

Dark banner green needs a light focus backing for readable contrast. No release/version change. CI is not awaited and this PR is not auto-merged, per user request.

[greptile-apps]

Greptile Summary

This change introduces two focused-control readability regressions. Selected labels in several pickers now use a dark green foreground on a dark selection bar, and active question tabs use theme accents on a fixed near-white backing that is unreadable in Ocean Dark and Monokai.

Merge safety: not safe to merge until these focused controls use accessible color pairings.

Confidence Score: 3/5

The change is not safe to merge because two independently reproduced display defects make focused interactive choices difficult to read.

Two independent, execution-confirmed failures affect visible keyboard-navigation controls.

Files Needing Attention: src/cortex-core/src/style.rs and src/cortex-tui/src/views/question_prompt.rs need accessible focused-state color treatments.

T-Rex Logs

What T-Rex did

• T-Rex produced proof for a posted P1 finding, and the reviewer can consult the corresponding review comment for details.
• T-Rex produced proof for another posted P1 finding, with the associated review comment detailing the finding.
• T-Rex produced proof for a third posted P1 finding, linked to its review comment with the finding details.
• T-Rex executed the pr40_selection_contrast_validation test and verified the current-PR output matched expectations, and the test passed.
• T-Rex ran the pr40_question_tab tests, including contrast validation, and confirmed the contrasts were below 4.5:1 per the validator, with the independent calculation matching the results.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "fix(tui): replace violet accent with ban..." | Re-trigger Greptile}