Improve patterns guide with concrete examples and safe area insets

- Rename "host styling" section to "host context" and add safe area
  insets handling
- Replace abstract `renderLoadingUI`/`renderFinalUI` example with
  concrete code preview pattern for `ontoolinputpartial`
- Rename callback parameter `params` → `ctx` for clarity across all
  `onhostcontextchanged` handlers
- Add CSS snippet for fullscreen border-radius handling
- Expand display mode example to show `availableDisplayModes` check
- Fix minor typos and wording consistency

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

src/react/useApp.tsx

@@ -102,8 +102,8 @@ export interface AppState {
  *       app.onerror = (error) => {
  *         console.log("Error:", error);
  *       };
- *       app.onhostcontextchanged = (params) => {
- *         setHostContext((prev) => ({ ...prev, ...params }));
+ *       app.onhostcontextchanged = (ctx) => {
+ *         setHostContext((prev) => ({ ...prev, ...ctx }));
  *       };
  *     },
  *   });