docs: update documentation to OKF v0.1 format

docs/guides/admin-categories.md

@@ -3,7 +3,7 @@ type: guide
 title: Admin — Categories
 description: How an admin lists, creates, edits, and deletes challenge categories from the /admin/categories page, including system-row protection and challenge-attached protection.
 tags: [guide, admin, categories, tester]
-timestamp: 2026-07-22T12:00:00Z
+timestamp: 2026-07-22T18:37:00Z
 ---
 
 # When this view is available
@@ -89,6 +89,29 @@ side-nav (categories are also embedded inside the General settings page).
    file is selected, the new icon is uploaded first and the returned
    `publicUrl` replaces `iconPath` in the same update.
 
+### Edit prefill mechanics
+
+When the parent sets `open=true` + `mode='edit'` + a `category`, a
+constructor `effect()` runs `syncCategoryForm(form, mode, category)`
+which:
+
+* writes `name`, `abbreviation`, `description` via per-control
+  `setValue(..., { emitEvent: false })`,
+* marks every control pristine + untouched so validators do not flash
+  errors on a freshly opened modal,
+* returns `{ abbreviationReadonly: category.isSystem === true,
+  iconPreview: category.iconPath || null }` which the effect pushes
+  into `abbreviationReadonly` and `iconPreview` signals and clears
+  `iconFile`,
+* and triggers `ChangeDetectorRef.markForCheck()` so the OnPush view
+  repaints the freshly-patched `<input>` / `<textarea>` values.
+
+Switching back to `mode='create'` with `category=null` re-invokes the
+helper, which resets every control to `''` and unlocks the abbreviation
+input. `syncCategoryForm` is exported from the modal file so it can be
+exercised in isolation by
+`tests/frontend/admin-categories-form-modal.spec.ts`.
+
 ## Delete behavior
 
 * **System rows:** the delete modal renders