feat: browse your own shares as a peer receives them

The window could show any peer's share tree but never this account's, so
"am I actually sharing that?" had no answer short of asking a friend to
browse you. The tree is already there; what was missing was feeding it
from our own index.

B opens it — the same collapsible tree, filter, and navigation the peer
browse offers, built from `Shares::directories()`, which is the listing
GetShareFileList answers with. So what it shows is what the network sees,
not what the config meant to share. r re-scans the disk rather than
re-asking a peer, o steps across to the folders that feed it, and nothing
here queues a download from ourselves.

Attached to a daemon the index lives on the daemon's host, so it comes
back over a new `shares.files` method rather than being scanned locally;
`shares files [--filter]` prints the same listing headless, through the
`emit_listing` that `browse` now shares with it — the two are only
comparable while they are built the same way.

Two allocation fixes came out of the review, both on paths this reaches:
the daemon hands its listing to the DTO instead of deep-copying one it
already owns, and `Shares::directories()` allocates its map key once per
folder rather than once per file, which the peer-facing serve path has
been paying for all along.

soulseek-rs-lib/src/shares/mod.rs

@@ -146,14 +146,18 @@ impl Shares {
                 .virtual_path
                 .rsplit_once('\\')
                 .unwrap_or(("", file.virtual_path.as_str()));
-            by_dir
-                .entry(dir.to_string())
-                .or_default()
-                .push(SharedFileEntry {
-                    name: base.to_string(),
-                    size: file.size,
-                    attributes: file.attributes.clone(),
-                });
+            let entry = SharedFileEntry {
+                name: base.to_string(),
+                size: file.size,
+                attributes: file.attributes.clone(),
+            };
+            // Looked up before inserting: `entry()` would allocate the key
+            // for every file, and a folder holds many.
+            if let Some(files) = by_dir.get_mut(dir) {
+                files.push(entry);
+            } else {
+                by_dir.insert(dir.to_string(), vec![entry]);
+            }
         }
         by_dir
             .into_iter()