fix(control-ui): polish tweakcn theme imports

Summary:
- Improve Control UI tweakcn theme import parsing and labeling.
- Apply imported theme names consistently across appearance controls.
- Document tweakcn share link and slug import flows.

Verification:
- pnpm test ui/src/ui/custom-theme.test.ts ui/src/ui/views/config.browser.test.ts ui/src/ui/views/config-quick.test.ts ui/src/ui/app-settings.test.ts ui/src/ui/storage.node.test.ts
- pnpm check:changed
- pnpm --dir ui build

docs/web/control-ui.md

@@ -82,6 +82,12 @@ The Control UI can localize itself on first load based on your browser locale. T
 - The selected locale is saved in browser storage and reused on future visits.
 - Missing translation keys fall back to English.
 
+## Appearance themes
+
+The Appearance panel keeps the built-in Claw, Knot, and Dash themes, plus one browser-local tweakcn import slot. To import a theme, open [tweakcn themes](https://tweakcn.com/themes), choose or create a theme, click **Share**, and paste the copied theme link into Appearance. The importer also accepts `https://tweakcn.com/r/themes/<id>` registry URLs, editor URLs like `https://tweakcn.com/editor/theme?theme=amethyst-haze`, relative `/themes/<id>` paths, raw theme IDs, and default theme names such as `amethyst-haze`.
+
+Imported themes are stored only in the current browser profile. They are not written to gateway config and do not sync across devices. Replacing the imported theme updates the one local slot; clearing it switches the active theme back to Claw if the imported theme was selected.
+
 ## What it can do (today)
 
 <AccordionGroup>