Theme: тесты (unit/widget) для theme pipeline

[ZhuchkaTriplesix]

Summary

Тестовая инфраструктура для системы тем: unit + widget tests, golden/fixtures.

Unit tests

Модуль	Тесты
JSONC preprocessor (#39)	comments, trailing comma, fixtures Dracula
Color parser (#53)	HEX variants
Deep merge (#45)	3-layer merge
QueryaTheme.fromDefaults (#38)	dark == legacy colors
VS Code map (#44)	key → token

Widget tests

• QueryaApp pump: switch ThemeController preset → expect ColorScheme.primary change
• QueryEditorTab with mock QueryaCodeEditor — font size from settings

Fixtures

test/fixtures/themes/
  dracula.json
  one-dark.json
  invalid-trailing-comma.jsonc

Acceptance criteria

• flutter test green in CI
• Coverage note в PR template (optional threshold не обязателен)

Зависимости

• Параллельно с Theme: модели QueryaWorkbenchTheme и QueryaEditorTheme #38–Theme: пользовательские color overrides (deep merge) #45
• Epic: [Epic] Система тем и стилей Querya Desktop #37

Оценка

~2 дня

[ZhuchkaTriplesix]

Delivered on dev in the Theme system milestone. See docs/theme.md and PRs #66, #74–#83.