android: a streamed file copied into a folder, the listening port, the dev build's diagnostics

The now-playing sheet's streamed track can be copied into a folder picked
on the phone, as the file it is — the cache written out, nothing downloaded
again, no library row (copy_stream_to_folder, which the desktop has had a
command for and no UI). You → Sharing carries the listening port, what
peers dial when they download from you: unset is the client's own 50300,
the app asks the router to forward it over UPnP and falls back to the
server relaying, and a change applies on the next connect. The dev flavour
gets the desktop's debug overlay as a screen under This phone — CPU,
memory, the network, the stream and its loudness, what the pre-buffer is
fetching, the size of the play history — behind devBuild in util.js, which
is Vite's own DEV flag, so the embedded build the phone keeps has no row
to tap.

src/mobile/Mobile.svelte

@@ -32,6 +32,7 @@
     import Thread from "./screens/Thread.svelte";
     import You from "./screens/You.svelte";
     import Transfers from "./screens/Transfers.svelte";
+    import Diagnostics from "./screens/Diagnostics.svelte";
     import { transfers, bindTransfers, active as activeTransfers } from "./transfers.svelte.js";
     import { bindHome } from "./home.svelte.js";
     import Stub from "./screens/Stub.svelte";
@@ -73,6 +74,7 @@
             case "thread": return { title: display(e.c.username), kicker: "Chat", flush: true };
             case "you": return { title: "You", kicker: "You" };
             case "transfers": return { title: "Transfers", kicker: "Downloads" };
+            case "diag": return { title: "Diagnostics", kicker: "Dev build" };
             case "import": return { title: "Import", kicker: importer.draft?.source_name || "Playlist" };
             case "browse": return { title: e.c.username, kicker: "Their files" };
             case "peer": return { title: e.c.username, kicker: "On Soulseek" };
@@ -251,6 +253,8 @@
                         <You />
                     {:else if entry.n === "transfers"}
                         <Transfers />
+                    {:else if entry.n === "diag"}
+                        <Diagnostics />
                     {:else}
                         <Stub kind={entry.n} />
                     {/if}