refactor: trim custom-endpoint comments and share synthetic provider const

Review pass: shorten verbose doc comments across the custom-endpoint code,
merge duplicate imports, share one synthetic provider entry between the keys
section and setup dialog, collapse the duplicated connect-editor block, revert
an unrelated settings-route drive-by.

app/api/settings/[section]/route.ts

@@ -128,6 +128,6 @@ export async function PUT(request: Request, ctx: { params: Promise<{ section: st
 	// Read-modify-write ONE section of the record; everything else rides
 	// along untouched (other sections, unknown keys — sanitize preserves).
 	const next = { ...record, [section]: sectionOut };
-	const saved = writeSettingsRecord(next);
-	return Response.json({ settings: saved });
+	writeSettingsRecord(next);
+	return Response.json({ settings: next });
 }