feat(web): OLED Black theme + per-appearance custom colors (#937)

* test: reproduce issue #866

* feat(web): OLED Black theme + per-appearance custom colors (closes #866)

Add an explicit OLED Black appearance (true #000 canvas, border-based
elevation) alongside system/dark/light, and a curated "key color"
customizer. Each key color (background, surface, text, hint, accent,
border, user bubble) cascades to its --app-* tokens and is stored per
appearance so a color tuned for light never leaks onto pure black.

via [HAPI](https://hapi.run)

Co-Authored-By: HAPI <noreply@hapi.run>

---------

Co-authored-by: HAPI <noreply@hapi.run>

web/src/lib/locales/en.ts

@@ -513,7 +513,19 @@ export default {
   'settings.display.appearance': 'Appearance',
   'settings.display.appearance.system': 'Follow System',
   'settings.display.appearance.dark': 'Dark',
+  'settings.display.appearance.oled': 'OLED Black',
   'settings.display.appearance.light': 'Light',
+  'settings.display.themeColors.title': 'Custom colors',
+  'settings.display.themeColors.description': 'Applies to the current appearance. Switch appearance to customize each one separately.',
+  'settings.display.themeColors.reset': 'Reset',
+  'settings.display.themeColors.resetAll': 'Reset all',
+  'settings.display.themeColors.key.background': 'Background',
+  'settings.display.themeColors.key.surface': 'Cards & surfaces',
+  'settings.display.themeColors.key.text': 'Text',
+  'settings.display.themeColors.key.hint': 'Muted text',
+  'settings.display.themeColors.key.accent': 'Accent & links',
+  'settings.display.themeColors.key.border': 'Borders',
+  'settings.display.themeColors.key.userBubble': 'Your message bubble',
   'settings.display.fontSize': 'Font Size',
   'settings.display.terminalFontSize': 'Terminal Font Size',
   'settings.display.sessionPreviewLimit': 'Sessions Before Folding',

web/src/lib/locales/zh-CN.ts

@@ -517,7 +517,19 @@ export default {
   'settings.display.appearance': '外观',
   'settings.display.appearance.system': '跟随系统',
   'settings.display.appearance.dark': '深色',
+  'settings.display.appearance.oled': 'OLED 纯黑',
   'settings.display.appearance.light': '浅色',
+  'settings.display.themeColors.title': '自定义配色',
+  'settings.display.themeColors.description': '应用于当前外观。切换外观可分别自定义每种配色。',
+  'settings.display.themeColors.reset': '重置',
+  'settings.display.themeColors.resetAll': '全部重置',
+  'settings.display.themeColors.key.background': '背景',
+  'settings.display.themeColors.key.surface': '卡片与表面',
+  'settings.display.themeColors.key.text': '文字',
+  'settings.display.themeColors.key.hint': '次要文字',
+  'settings.display.themeColors.key.accent': '强调与链接',
+  'settings.display.themeColors.key.border': '边框',
+  'settings.display.themeColors.key.userBubble': '你的消息气泡',
   'settings.display.fontSize': '字体大小',
   'settings.display.terminalFontSize': '终端字体大小',
   'settings.display.sessionPreviewLimit': '会话折叠阈值',