Restore educational comments in basic-server-* examples

PR #184 accidentally removed/reverted educational comments from
`basic-server-react` and `basic-server-vanillajs` that were established
in PR #182. This restores consistency with the other basic-server
examples (Vue, Svelte, Preact, Solid).

Changes:
- Add back the three-comment pattern explaining two-part registration
- Restore full tool description ("as an ISO 8601 string")
- Change vanillajs to return plain text instead of JSON (matching #182)
- Use local `resourceUri` variable instead of top-level constant

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/basic-server-react/server.ts

@@ -17,21 +17,27 @@ export function createServer(): McpServer {
     version: "1.0.0",
   });
 
+  // Two-part registration: tool + resource, tied together by the resource URI.
   const resourceUri = "ui://get-time/mcp-app.html";
 
+  // Register a tool with UI metadata. When the host calls this tool, it reads
+  // `_meta[RESOURCE_URI_META_KEY]` to know which resource to fetch and render
+  // as an interactive UI.
   registerAppTool(server,
     "get-time",
     {
       title: "Get Time",
-      description: "Returns the current server time.",
+      description: "Returns the current server time as an ISO 8601 string.",
       inputSchema: {},
       _meta: { [RESOURCE_URI_META_KEY]: resourceUri },
     },
     async (): Promise<CallToolResult> => {
-      return { content: [{ type: "text", text: new Date().toISOString() }] };
+      const time = new Date().toISOString();
+      return { content: [{ type: "text", text: time }] };
     },
   );
 
+  // Register the resource, which returns the bundled HTML/JavaScript for the UI.
   registerAppResource(server,
     resourceUri,
     resourceUri,