[Sepush]

The docs site put the theme in the path (/zh-CN/os-theme/components/button). The theme is the reader's preference, not part of the page, and having it in the URL means:

• a shared link carries the sender's theme, not the reader's;
• the choice is lost when someone comes back through the root address;
• every theme switch adds a history entry, so Back flips the theme;
• each page has three URLs (light, dark, os-theme).

This PR keeps the language in the path and moves the theme out of it.

• URLs: /:lang/docs/... and /:lang/components/.... Menus, search, header and home links follow.
• Preference: a small useColorMode composable in demo/utils/color-mode.js, shaped after VueUse's: mode is light, dark or auto (follow the OS, via useOsTheme), state is the theme in effect. Only an explicit pick is saved in localStorage (naive-ui-theme); with nothing saved the site follows the OS and updates when the OS switches.
• One-step switch: like VueUse's disableTransition, transitions are turned off while the colors change, so the page switches at once instead of every component fading on its own.
• Picker: the header button is now a popselect: Light, Dark or System. On mobile it is a submenu at the top of the menu. Choosing System clears the saved choice.
• No flash on reload: a small inline script in index.html sets color-scheme on <html> before the first paint, so the page has a dark canvas until the styles load.
• Tabs: a change in one tab applies to the other open tabs (storage event).
• Old links keep working: /:lang/(light|dark|os-theme)/... redirects to /:lang/..., keeping the query and hash. The theme in an old link is ignored, so someone else's link doesn't change the reader's setting.

Old naiveui.com/<lang>/os-theme/... URLs in the library source (for example in useMessage's error message) still work through the redirect, so this PR leaves them alone.

Tested in Chromium (headless) against a production build of the site: root redirect by browser language, legacy links with query and hash, OS dark/light with nothing saved, picking each option and reloading, the page already dark with all JS blocked, tab sync, Back after navigation, and the mobile menu. With the CPU throttled 4×, switching on the Data Table page finishes in about 1.3s, against about 2.0–2.5s on naiveui.com.

The last commit is the same change as #8243 (vue-tsc template scope errors in the image and avatar demos), so the docs lint job passes. It is identical to that PR, so the two merge cleanly.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.56%. Comparing base (7ef48da) to head (2c29ce8).
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Additional details and impacted files

@@            Coverage Diff             @@
##             docs    #8245      +/-   ##
==========================================
- Coverage   63.56%   63.56%   -0.01%     
==========================================
  Files         878      878              
  Lines       27467    27487      +20     
  Branches     8730     8524     -206     
==========================================
+ Hits        17460    17471      +11     
- Misses       9793     9803      +10     
+ Partials      214      213       -1     

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