feat(gpui_widgets): add olive theme system with runtime switching

W7 of the oak task list. OakTheme::olive_dark/olive_light translate oak's
palette.ini files into a structured theme (window/base/alternate/accent/
link/disabled plus a derived border). apply_theme swaps gpui's GlobalColors
so every widget reading cx.default_colors() re-themes immediately, and
stores the full theme in a ThemeGlobal. Palette contrast and mapping are
unit-tested, with a gpui::test verifying the runtime switch;
examples/themes.rs toggles between the two themes live. 101 widget tests
pass.

docs/zh/oak-app-rewrite.md

@@ -126,8 +126,14 @@
 
 ## W7. 主题系统
 
-- [ ] 设计系统：把 oak 的 olive-dark/olive-light QSS 翻译成 gpui
+- [x] 设计系统：把 oak 的 olive-dark/olive-light QSS 翻译成 gpui
   的 `Colors`/样式结构，支持运行期切换。
+  > `gpui_widgets::theme`：`OakTheme::olive_dark()/olive_light()`
+  > 从 `oak/app/ui/style/olive-*/palette.ini` 提取（window/base/
+  > accent/link/disabled 等 + 派生 border）；`apply_theme` 同时设置
+  > gpui 的 `GlobalColors`（所有读 `cx.default_colors()` 的控件立即
+  > 换肤）与扩展的 `ThemeGlobal`。调色板对比度/映射单测 +
+  > `#[gpui::test]` 运行期切换验证。`examples/themes.rs`。
 
 ## 顺序与验收