iOS: Audio messages, file attachments, and caching

Audio messages, file attachments, and three-layer caching added to the iOS app.

• Record audio with waveform preview and inline playback
• Send photos, documents, and PDFs with attachment grid layout
• Three-layer cache: images, link previews, and messages persist locally
• Apple Handoff to continue conversations between iPhone and Mac
• Swipe right to reply, tap and hold for emoji reactions
• In-conversation search and date headers for long threads
• iPad split view: browse inbox and read a thread side by side