Add catalog item images with thumbnail preview in wizard

Each catalog item now supports one optional image (jpg/jpeg/png/gif/webp,
max 10 MB). Uploading generates a 200x200 JPEG thumbnail automatically via
SixLabors.ImageSharp. Images are stored in Azure Blob Storage under a new
catalogimages container, keyed by {companyId}/catalog/{itemId}/.

- CatalogItem entity: ImagePath + ThumbnailPath (nullable string fields)
- Migration: AddCatalogItemImages applied
- ICatalogImageService / CatalogImageService: upload, thumbnail generation,
  delete; old blobs replaced atomically on re-upload
- CatalogItemsController: Create/Edit accept optional IFormFile image;
  Image(id, thumbnail) action serves blobs with [Authorize] so wizard users
  can load thumbnails without CanManageProducts policy
- Catalog index (_CategoryNode): 40x40 thumbnail (or placeholder icon)
  left of each item name
- Details view: image card in right column with click-to-full-size link
- Create/Edit views: file picker with live preview; Edit shows current
  thumbnail with Remove checkbox
- Wizard (item-wizard.js): thumbnails in product list with hover preview
  that follows the cursor (showCatalogPreview / moveCatalogPreview);
  fixed Bootstrap d-flex !important bug that broke the filter box by
  moving flex layout to an inner wrapper div

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Helpers/HelpKnowledgeBase.cs

@@ -916,9 +916,12 @@ public static class HelpKnowledgeBase
         **How to add a catalog item:**
         1. Go to [Catalog Items](/CatalogItems) → "New Item"
         2. Enter name, category, and the all-in price (including your labor and margin — nothing will be added on top)
-        3. Save
+        3. Optionally upload an image in the Item Image section (jpg/jpeg/png/gif/webp, max 10 MB — a 200×200 thumbnail is generated automatically)
+        4. Save
 
-        Catalog items can be selected in the quote/job wizard as an alternative to the full calculated or custom item workflow.
+        **Item images:** Each catalog item supports one optional image. Upload or replace it on the item's Edit page. When no image is set, a gray placeholder icon appears instead. Images appear as thumbnails in the catalog list and in the quote/job item wizard. Hovering over a thumbnail in the wizard shows a larger preview near the cursor so staff can quickly confirm the right part.
+
+        Catalog items can be selected in the quote/job wizard as an alternative to the full calculated or custom item workflow. The wizard's product list includes a search/filter box and shows thumbnails next to each item name for visual identification.
 
         **Saving to catalog directly from the item wizard (Save-to-Catalog step):**
         When a user completes a Calculated or AI Photo Quote item in the wizard, a final optional step appears: "Save to Product Catalog." This lets them create a reusable catalog entry from the item they just configured — without navigating to the Catalog Items page separately.