feat(media): extract media details into reusable modal component

- add `MediaDetailsModal.client.js` with support for `media={…}` or `slug="…"` props
- add `GET /zen/api/media/by-slug/:slug` route for slug-based lookup
- refactor `MediaPage.client.js` to use the new modal instead of inline details panel
- export `MediaDetailsModal` as `./features/media/details-modal` in package.json
- update `BlockEditor` image block to open `MediaDetailsModal` for media editing
- update media feature README to document new component and route

package.json

@@ -94,6 +94,9 @@
     "./features/media/picker": {
       "import": "./dist/features/media/components/MediaPicker.client.js"
     },
+    "./features/media/details-modal": {
+      "import": "./dist/features/media/components/MediaDetailsModal.client.js"
+    },
     "./features/provider": {
       "import": "./dist/features/provider/index.js"
     },