fix(web): media placeholder reads LOADING MEDIA, not MISSING MEDIA

The ::after placeholder text on .dt-bevel-media and empty card bodies is
mostly seen while lazy media loads, where MISSING MEDIA reads as 'the
store is broken'. Neutral loading wording instead. Bump to 0.5.1.

Revert with: git revert <this-sha>

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/web/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@dangerousthings/web",
-  "version": "0.5.0",
+  "version": "0.5.1",
   "description": "Web CSS theme for the Dangerous Things design system — bevels, glows, form styles",
   "license": "MIT",
   "author": {