fix: preserve textAlign/verticalAlign/containerId through REST round-trip

ElementSharedFieldsSchema in server.ts was silently stripping textAlign,
verticalAlign, and containerId on every POST/PATCH, causing bound text
inside containers to lose centering after a sync round-trip.

- Add missing fields to ElementSharedFieldsSchema (server.ts)
- Add textAlign?, verticalAlign?, containerId? to ServerElement (types.ts)
- Set textAlign: "center", verticalAlign: "top" on MCP subtitle elements (index.ts)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/server.ts

@@ -487,6 +487,10 @@ const ElementSharedFieldsSchema = z.object({
   endBinding: z.any().nullable().optional(),
   boundElements: z.any().nullable().optional(),
   elbowed: z.boolean().optional(),
+  // Text alignment properties (required for bound text inside containers)
+  textAlign: z.string().optional(),
+  verticalAlign: z.string().optional(),
+  containerId: z.string().nullable().optional(),
   // Image element properties
   fileId: z.string().optional(),
   status: z.string().optional(),