fix(theme): replace Gruvbox with Midnight, fix accent/info hue collisions and hardcoded colors

- Replace Gruvbox preset with Midnight (deep blue-purple, violet accent)
- Fix accent/info hue collisions in Nord, Monokai, Dracula, Solarized
- Fix Solarized error/warning collision (error-h 25->5) and WCAG contrast
- Make --color-skip-refresh-* follow --color-warning-h dynamically
- Replace hardcoded rgba(24,144,255) in onboarding.css with --color-accent
- Replace hardcoded #00B87A in import modals with --color-success

locales/zh-CN.json

@@ -255,7 +255,7 @@
             "presets": "主题预设",
             "default": "默认",
             "nord": "Nord",
-            "gruvbox": "Gruvbox",
+            "midnight": "Midnight",
             "monokai": "Monokai",
             "dracula": "Dracula",
             "solarized": "Solarized",