feat(ui): 主题切换 + 语言持久化 + About 对话框

- 新增 light/dark 主题调色板与 Windows 注册表系统主题探测
- 偏好设置对话框：主题三选一 + 语言下拉(en/zh-CN/de)，即时生效
- 持久化 Language/Theme/AutoLockMinutes/MinimizeToTray 至 YAML 顶层
- 启动时按 config 中保存的语言初始化 i18n
- About 对话框展示 version/runtime/项目地址/许可证
- 全部硬编码颜色迁移到 themePalette，为深色模式做准备

internal/ui/app.go

@@ -10,12 +10,10 @@ import (
 	"time"
 
 	"gioui.org/app"
-	"gioui.org/font/gofont"
 	"gioui.org/layout"
 	"gioui.org/op"
 	"gioui.org/op/clip"
 	"gioui.org/op/paint"
-	"gioui.org/text"
 	"gioui.org/unit"
 	"gioui.org/widget"
 	"gioui.org/widget/material"
@@ -95,6 +93,8 @@ type appState struct {
 	hotkeyDialog *hotkeyDialog
 	hotkeyTarget *entry
 	tradesDialog *steamTradesDialog
+	prefsDialog  *preferencesDialog
+	aboutDialog  *aboutDialog
 
 	store   *store
 	saveErr string // surfaced in the top bar
@@ -102,6 +102,13 @@ type appState struct {
 	hkMgr *win32.HotkeyManager
 
 	toast toast
+
+	// themeMode is the user's current theme preference. drawFrame
+	// rebuilds the material.Theme when this changes.
+	themeMode themeMode
+	// theme is the cached material.Theme matching themeMode. nil forces a
+	// rebuild on the next frame.
+	theme *material.Theme
 }
 
 // snapshotEntries returns a freshly serialized slice of config entries.
@@ -119,9 +126,6 @@ func (st *appState) snapshotEntries() []config.Entry {
 func loop(w *app.Window, configPath string) error {
 	const fn = "internal.ui.loop"
 
-	th := material.NewTheme()
-	th.Shaper = text.NewShaper(text.WithCollection(gofont.Collection()))
-
 	state := &appState{}
 	state.list.Axis = layout.Vertical
 
@@ -146,6 +150,12 @@ func loop(w *app.Window, configPath string) error {
 		state.absorbConfig(cfg)
 	}
 
+	// Seed the theme cache from the stored preference. Empty / unknown
+	// values resolve to "system" which buildTheme then maps to light or
+	// dark via the OS-specific systemPrefersDark probe.
+	_, themePref, _, _ := state.store.Preferences()
+	state.themeMode = normalizeThemeMode(themePref)
+
 	// Spin up the global hotkey manager and register whatever the user
 	// already had configured. Failures are non-fatal (logged + the row
 	// just won't fire).
@@ -170,7 +180,15 @@ func loop(w *app.Window, configPath string) error {
 			return e.Err
 		case app.FrameEvent:
 			gtx := app.NewContext(&ops, e)
-			drawFrame(gtx, th, state, w)
+			if state.theme == nil {
+				th, pal := buildTheme(state.themeMode)
+				state.theme = th
+				activePalette = pal
+			}
+			// Paint the window background using the active palette so
+			// dark mode does not show through as the Gio default grey.
+			fillBackground(gtx, activePalette.Background)
+			drawFrame(gtx, state.theme, state, w)
 			e.Frame(gtx.Ops)
 		}
 	}
@@ -320,8 +338,11 @@ func drawFrame(gtx layout.Context, th *material.Theme, st *appState, w *app.Wind
 				st.setPwDialog = newSetPasswordDialog()
 			case settingsActionImportLegacy:
 				st.importDialog = newImportLegacyDialog()
+			case settingsActionPreferences:
+				lang, theme, _, _ := st.store.Preferences()
+				st.prefsDialog = newPreferencesDialog(lang, normalizeThemeMode(theme))
 			case settingsActionAbout:
-				// TODO: about dialog (next phase).
+				st.aboutDialog = newAboutDialog()
 			}
 			w.Invalidate()
 		} else {
@@ -329,6 +350,27 @@ func drawFrame(gtx layout.Context, th *material.Theme, st *appState, w *app.Wind
 		}
 	}
 
+	if st.prefsDialog != nil {
+		return st.prefsDialog.Layout(gtx, th, func(r preferencesResult) {
+			if !r.cancel {
+				_, _, autoLock, minToTray := st.store.Preferences()
+				st.store.SetPreferences(r.language, string(r.theme), autoLock, minToTray)
+				i18n.SetLanguage(r.language)
+				st.themeMode = r.theme
+				st.theme = nil // force rebuild on next frame
+			}
+			st.prefsDialog = nil
+			w.Invalidate()
+		})
+	}
+
+	if st.aboutDialog != nil {
+		return st.aboutDialog.Layout(gtx, th, func() {
+			st.aboutDialog = nil
+			w.Invalidate()
+		})
+	}
+
 	if st.importDialog != nil {
 		return st.importDialog.Layout(gtx, th, func(r importLegacyResult) {
 			if !r.cancel && r.cfg != nil {
@@ -427,7 +469,7 @@ func drawFrame(gtx layout.Context, th *material.Theme, st *appState, w *app.Wind
 					return layout.Dimensions{}
 				}
 				lbl := material.Body2(th, msg)
-				lbl.Color = color.NRGBA{R: 0xc0, A: 0xff}
+				lbl.Color = activePalette.ErrorFg
 				return layout.Inset{Top: unit.Dp(4)}.Layout(gtx, lbl.Layout)
 			}),
 			layout.Rigid(layout.Spacer{Height: unit.Dp(8)}.Layout),
@@ -487,7 +529,7 @@ func entryRow(gtx layout.Context, th *material.Theme, en *entry) layout.Dimensio
 						}),
 						layout.Rigid(func(gtx layout.Context) layout.Dimensions {
 							lbl := material.H6(th, en.Code)
-							lbl.Color = color.NRGBA{R: 0x10, G: 0x70, B: 0xff, A: 0xff}
+							lbl.Color = activePalette.RingFg
 							return lbl.Layout(gtx)
 						}),
 					)