Fix Fountain.fm episode cards: use Tauri HTTP plugin for CORS bypass

- Import fetch from @tauri-apps/plugin-http (browser fetch was CORS-blocked)
- Add fountain.fm to Tauri HTTP capability scope
- Extract og:audio meta tag for reliable audio URL resolution
- Parse OG title into show name + episode title

src-tauri/capabilities/default.json

@@ -14,7 +14,8 @@
         { "url": "https://nostr.build/**" },
         { "url": "https://void.cat/**" },
         { "url": "https://nostrimg.com/**" },
-        { "url": "https://relay.vertexlab.io/**" }
+        { "url": "https://relay.vertexlab.io/**" },
+        { "url": "https://fountain.fm/**" }
       ]
     },
     "dialog:default",