Unify attachment display and enable file uploads on all platforms (#1501)

Attachment pills were drawn by four separate style sets, so files,
images, GitHub items, and reviews rendered at different heights in the
same tray. Move the frame, the two-line label, and the image thumbnail
into shared components so every attachment lines up at one height,
across both the composer and sent messages.

Sent-message images now open the lightbox, and file uploads work on
iOS and Android via expo-document-picker instead of being gated to
desktop.

packages/app/package.json

@@ -66,6 +66,7 @@
     "expo-constants": "~18.0.9",
     "expo-crypto": "^15.0.8",
     "expo-dev-client": "^6.0.15",
+    "expo-document-picker": "~14.0.8",
     "expo-file-system": "~19.0.17",
     "expo-haptics": "~15.0.7",
     "expo-image": "~3.0.10",