fix(control-ui): allow configured chat message width

Adds validated gateway.controlUi.chatMessageMaxWidth support for grouped Control UI chat messages, carries it through the Gateway bootstrap payload into UI state, applies it as a CSS custom property, and documents the setting while preserving the existing default width.

Fixes #67935.

Validation:
- Targeted config, gateway, and Control UI tests passed locally.
- Config schema/docs checks passed.
- Testbox changed-file gate passed.
- GitHub CI and security checks are green on cea25a4ca9.

docs/web/control-ui.md

@@ -247,6 +247,22 @@ Use `trusted` only when the embedded document genuinely needs same-origin behavi
 
 Absolute external `http(s)` embed URLs stay blocked by default. If you intentionally want `[embed url="https://..."]` to load third-party pages, set `gateway.controlUi.allowExternalEmbedUrls: true`.
 
+## Chat message width
+
+Grouped chat messages use a readable default max-width. Wide-monitor deployments can override it without patching bundled CSS by setting `gateway.controlUi.chatMessageMaxWidth`:
+
+```json5
+{
+  gateway: {
+    controlUi: {
+      chatMessageMaxWidth: "min(1280px, 82%)",
+    },
+  },
+}
+```
+
+The value is validated before it reaches the browser. Supported values include plain lengths and percentages such as `960px` or `82%`, plus constrained `min(...)`, `max(...)`, `clamp(...)`, `calc(...)`, and `fit-content(...)` width expressions.
+
 ## Tailnet access (recommended)
 
 <Tabs>