Preserve control backgrounds in focus tints

[thunderballfists]

Focused primary and error buttons lost their variant colors because TextUI set a translucent background instead of blending a tint over the native background. Both the default focus cue and compact editable-control cue now use background-tint, preserving native and author backgrounds. The existing 15% and 25% strengths are unchanged; applications can adjust or disable them in TCSS.

Adds regression coverage for variant buttons and author backgrounds across runtime compact-mode toggles, documents the override, and updates reviewed macOS/Linux showcase baselines. Snapshot text and geometry are unchanged.

Validation:

• Full headless suite: 396 passed; styling suite: 24 passed. Four new regressions failed before the fix.
• Visual snapshot comparison: 6 passed on macOS and 6 passed in a Python 3.12 Linux container; rendered variant colors reviewed.
• Poetry metadata/lock check, wheel and source build, Pyflakes, and diff whitespace check passed.

Closes #91.