docs: document CC_SWITCH_GDK_BACKEND Linux Wayland escape hatch

Document the opt-in CC_SWITCH_GDK_BACKEND environment variable (added in
#4351) that overrides the AppImage's hardcoded GDK_BACKEND=x11, letting
Wayland+NVIDIA users switch back to native Wayland when the webview goes
click-dead and black-screens on resize. The override is generic, so
tiling-Wayland users can set it to x11 for the inverse input bug.

- CHANGELOG.md: new Unreleased/Fixed entry (#4351, fixes #4350)
- README.md / README_ZH / README_JA / README_DE: FAQ entry across all locales
- docs/user-manual/{zh,en,ja}/5-faq/5.2-questions.md: Linux troubleshooting entry

README.md

@@ -284,6 +284,19 @@ Add an official provider from the preset list. After switching to it, run the Lo
 
 </details>
 
+<details>
+<summary><strong>Linux (Wayland + NVIDIA): clicks don't register and the window black-screens on resize</strong></summary>
+
+The AppImage forces `GDK_BACKEND=x11` (XWayland) to avoid a historical native-Wayland crash. On newer Wayland + NVIDIA setups this can leave the web content area unclickable (the title-bar buttons still work) and black-screen on resize. Launch with the opt-in escape hatch to switch back to native Wayland:
+
+```bash
+CC_SWITCH_GDK_BACKEND=wayland ./CC-Switch-*.AppImage
+```
+
+If you launch from a desktop icon, add it to the `.desktop` `Exec=` line (e.g. `env CC_SWITCH_GDK_BACKEND=wayland /path/to/AppImage`) or set it in your session environment. The variable is generic: on tiling Wayland compositors (sway/Hyprland) where clicks don't register, try `CC_SWITCH_GDK_BACKEND=x11` instead. Leaving it unset keeps the default behavior.
+
+</details>
+
 ## Documentation
 
 For detailed guides on every feature, check out the **[User Manual](docs/user-manual/en/README.md)** — covering provider management, MCP/Prompts/Skills, proxy & failover, and more.