feat(web): add windowed Settings app, separate from initial Config screen

The taskbar's gear icon reopened the full-page "Connect to Oikos" screen
even once already connected. Split that: Config.svelte stays as the
first-run/unconfigured screen; a new Settings app (windowed, like Tasks or
Operations) now handles in-session changes, with a section list (Connection,
Appearance) built to grow — future settings are one more entry, not a new
screen.

- pages/Settings.svelte: Connection (server URL/token/Authentik, reusing
  config.ts + oidc.ts) and Appearance (Terracotta/Carbon picker) sections.
- apps.ts: registered as a normal desktop app.
- Taskbar's gear button now opens the Settings window; removed the
  onOpenConnection prop threaded through App -> Desktop -> Taskbar, since
  Settings' "Forget saved connection" (clear config + reload) replaces it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/App.svelte

@@ -62,5 +62,5 @@
   />
 {:else}
   <Toaster />
-  <Desktop onOpenConnection={() => (configured = false)} />
+  <Desktop />
 {/if}