iOS: Share Extension so Buzz appears in the system share sheet

**Motivation**

On iOS, Buzz never appears in the system share sheet ("share" menu). When users share a photo, screenshot, or file from Photos, Files, Safari, or any other app, Buzz is not offered as a destination — so the only way to attach media is to open Buzz first, use the composer's attachment picker, and hunt for the file there.

This matters exactly when people already have the content in hand (a screenshot of something to ask an agent about, a photo to send to a DM). Sharing should be one tap from Photos/Files → Buzz, matching the flow of every other messaging app on iOS.

Note: attaching from *inside* the app is already well supported (attachment picker, paste, galleries). This request is only about the inbound system share path.

**Proposed solution**

Add an iOS Share Extension:

1. Register Buzz as a share target (`NSExtension` with `com.apple.share-services` in the iOS app) so it appears in the iOS share sheet for images, videos, and files.
2. On share, open (or deep-link into) the composer of a chosen DM/channel with the item attached, reusing the existing attachment upload pipeline.
3. If multiple items are shared at once, attach them in order.
4. (Nice-to-have, later: an Android `ACTION_SEND` intent filter for parity.)

**Alternatives considered**

- Copy/paste workaround (screenshot → paste image) — works in principle but iOS "Paste Image" is currently broken (#6299), and it's still several steps and clipboard-only.
- Opening the app and using the attachment picker — the current status quo; slower and doesn't handle "share from where I am" flows.

**Additional context**

None found searching the issue tracker for "share extension", "share sheet", "share target" and similar terms — this appears unreported. Related but distinct: #6299 (iOS Paste Image does nothing), #3313 (attachment picking inside the app, closed/shipped), #7139 (camera photos rejected for metadata).