feat: add Apple App Site Association for iOS universal links

Enables grain.social links to open directly in the native iOS app
for profile, gallery, and story URLs.

Co-Authored-By: Claude Opus 4.6 <[e-mail]>

static/.well-known/apple-app-site-association

···
+{
+  "applinks": {
+    "apps": [],
+    "details": [
+      {
+        "appID": "YN68LN9T7Z.social.grain.grain",
+        "paths": [
+          "/profile/*/gallery/*",
+          "/profile/*/story/*",
+          "/profile/*"
+        ]
+      }
+    ]
+  }
+}