feat(network): support custom relay servers

Add strict custom Iroh relay profiles with safe restart and rollback across the Rust core, Compose apps, and Apple apps. Preserve multi-relay invitations and fail closed on configuration or recovery mismatches.

README.md

@@ -50,6 +50,31 @@ mobile networks. If a direct path cannot be established, it can forward the
 same end-to-end encrypted connection through a relay. The relay forwards
 encrypted packets; it is not a VniDrop file store.
 
+### Custom relay servers
+
+VniDrop uses Iroh's public relay and discovery infrastructure by default. In
+**Settings → Network**, advanced users can instead configure up to eight custom
+HTTPS servers that implement Iroh's relay protocol. Custom mode is strict: the
+app uses only those relays and does not silently fall back to public relays or
+public discovery. Direct peer-to-peer paths remain available.
+
+Applying a relay change restarts VniDrop's network engine, so active transfers
+and shares must be stopped first. The app tests the new configuration and
+restores the previous one if it cannot connect. Invitations created for an old
+relay configuration may need to be shared again; stopped shares never expose
+their stale invitations. If a long relay profile makes an invitation too large
+for a QR code, use the native share action or export the invitation file.
+
+Relay credentials embedded in URLs are deliberately rejected and bearer-token
+authentication is not currently supported. A self-hosted relay must either
+accept the connecting endpoints or authorize their endpoint IDs independently;
+the current device ID is shown in **Settings → Network** for this purpose.
+Configure the same relay profile on participating devices. A custom relay needs
+a TLS certificate issued by a publicly trusted WebPKI certificate authority;
+private or enterprise CAs installed only in the operating system are not used
+in this version. For resilient deployments, configure at least two relays in
+different failure domains.
+
 ## Why Iroh and `iroh-blobs`?
 
 VniDrop combines a networking layer with its own sharing rules:
@@ -90,6 +115,7 @@ people, especially when using **Anyone with this transfer**.
 - Safe receive destinations that do not silently overwrite existing files
 - Native SwiftUI apps on iOS, iPadOS, and macOS; Compose apps on Android,
   Windows, and Linux
+- Strict custom HTTPS relay profiles with safe apply and rollback
 - Opt-in diagnostics with transfer contents, invitations, and file paths
   excluded