remove iframe.diy sandbox, nsite preview, and webxdc runtime

The iframe.diy-based sandbox infrastructure powered two features:
the nsite preview dialog (Run button on NsiteCard and AppHandlerContent)
and the webxdc embed (cartridge launcher + sandboxed iframe runtime
with kind 4932/20932 sync events).

Both are removed wholesale:

- iframe sandbox plumbing: SandboxFrame, src/lib/sandbox/*,
  iframeSubdomain, previewInjectedScript.
- nsite preview: NsitePreviewDialog, NsitePermissionManager,
  NsitePermissionPrompt, useNsiteSignerRpc, nsitePermissions,
  nsiteNostrProvider, NsitePlayerContext.
- webxdc runtime: WebxdcEmbed, Webxdc, GameControls, useWebxdc,
  webxdcMeta, public/cartridge.png, NOSTR_WEBXDC.md,
  @webxdc/types dependency, kindLabels/signerWithNudge entries
  for kinds 4932/20932.
- AppConfig: drop sandboxDomain, showWebxdc, feedIncludeWebxdc.
- extraKinds: drop kind 1063 webxdc entry; sidebar drops 'webxdc'.
- ComposeBox: .xdc uploads now flow through the generic file path
  (no UUID injection, no manifest extraction, .xdc removed from
  the file picker accept list).
- NoteContent / FileMetadataContent: webxdc branches removed; .xdc
  attachments fall through to the generic file card.
- LayoutContext / CenterColumnContext: only consumed by the removed
  fullscreen preview panels — deleted along with its provider in
  AppRouter.

NsiteCard keeps its rich link-preview card but loses the Run button
and preview dialog. AppHandlerContent keeps a kind 35128 `a`-tag
reference but replaces 'Run' with an external 'Open Site' link to
`<pubkeyB36><dTag>.nsite.lol`. The standard HTML iframe `sandbox`
attribute used by SpotifyEmbed/TweetEmbed/RedditEmbed is unrelated
to iframe.diy and stays.

package-lock.json

@@ -169,7 +169,6 @@
         "@types/react-dom": "^19.2.3",
         "@vitejs/plugin-react": "^6.0.1",
         "@webbtc/webln-types": "^3.0.0",
-        "@webxdc/types": "^2.1.2",
         "autoprefixer": "^10.4.20",
         "eslint": "^9.9.0",
         "eslint-plugin-react-hooks": "^5.1.0-rc.0",
@@ -7418,13 +7417,6 @@
         "url": "lightning:hello@getalby.com"
       }
     },
-    "node_modules/@webxdc/types": {
-      "version": "2.1.2",
-      "resolved": "https://registry.npmjs.org/@webxdc/types/-/types-2.1.2.tgz",
-      "integrity": "sha512-oklcyHvUXqCS5JwbPVaN8tt7nSB8ffRmyrUlVt0HeSn4kDyNE46oKSbw3KtrDzl530KHnm67LfcK/AjWbBoXUA==",
-      "dev": true,
-      "license": "unlicense"
-    },
     "node_modules/@xmldom/xmldom": {
       "version": "0.8.12",
       "resolved": "https://registry.npmjs.org/@xmldom/xmldom/-/xmldom-0.8.12.tgz",