Shared theme variable contract for tunacode and nextstep palettes

Shared theme variable contract for tunacode and nextstep palettes

Summary

Established a single variable schema for both Textual themes so base TCSS can stay theme-agnostic and avoid missing-variable runtime paths.

Changes

• Updated src/tunacode/constants.py:
  • Added THEME_VARIABLE_CONTRACT mapping variable names to palette keys.
  • Added _build_theme_variables() with fail-fast validation for missing palette keys.
  • Added missing shared palette keys to both UI_COLORS and NEXTSTEP_COLORS:
    • bevel_light
    • bevel_dark
    • scrollbar_thumb
    • scrollbar_track
  • Updated build_tunacode_theme() and build_nextstep_theme() to emit variables from the same contract.
  • Removed now-unused NEXTSTEP_COLORS entries (window_content, title_bar, title_bar_text).
• Added tests/unit/ui/test_theme_variable_contract.py to verify:
  • both palettes include all required keys,
  • both theme builders emit identical variable schemas,
  • variable values map correctly from each palette.

Verification

• uv run ruff check src/tunacode/constants.py tests/unit/ui/test_theme_variable_contract.py
• uv run pytest tests/unit/ui/test_theme_variable_contract.py -q

Result: pass.