Unify colour scheme and CSS as one Style list in settings UI (ticket 047)

The settings panel had two separate sections — "Colour scheme" (built-in
palettes) and "CSS" (imported/linked stylesheets) — even though the data
model (tickets 012/020) already treats both as equally-selectable themes
via custom_theme_id. Merge them into one "Style" section presenting a
single list: built-in scheme pills plus imported/linked entries, any of
which can be a palette swap or a full layout override, with copy that no
longer implies "colour scheme" and "CSS" are separate features.

Traced and manually verified (via a running dev server) that a
user-supplied stylesheet is not constrained to --c-* variable overrides:
import_css/get_css_content in src/api.rs and src/settings.rs store and
serve the raw content unmodified (no sanitization or selector scoping),
and applyCustomThemeCss injects it as a plain unscoped <link> tag — a
stylesheet with e.g. `flex-direction`/`font-size` rules on real selectors
applies exactly as authored, alongside the required --c-* declarations.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/lib/useSettingsPanel.ts

@@ -153,7 +153,8 @@ export function useSettingsPanel() {
     [guarded],
   )
 
-  // Every entry is always a selectable theme (ticket 020), so there's a single import path.
+  // Every entry is always a selectable style, on equal footing with the built-in colour schemes
+  // (ticket 020 unified the data model; ticket 047 unified the UI), so there's a single import path.
   const importCssEntry = useCallback(
     (filename: string, content: string) =>
       guarded(async () => {