feat(settings): reorganize the settings dialog and the model settings

Tabs Models, General, Drawing and About, each with a fixed title row.

The models tab becomes three pages: the user's providers with one status
each and the server's models; a searchable picker of all 24 providers in
four groups; and a provider page in three steps (connection, models,
test). A new provider gets its first suggested model, and a passing test
offers to use the model in the chat. Bedrock asks for one way of signing
in, with an optional session token for temporary access keys.

The composer's model picker lists every model, marking untested and
failed ones, and can open the provider picker. A chat error that offers
model settings opens the provider the request went to.

components/settings/settings-header.tsx

@@ -0,0 +1,8 @@
+/** A settings page's title row; the dialog's close button sits at its end */
+export function SettingsHeader({ children }: { children: React.ReactNode }) {
+    return (
+        <header className="flex h-14 shrink-0 items-center gap-1.5 pr-14 pl-6 text-[15px]">
+            {children}
+        </header>
+    )
+}