ci: run soulfind e2e tests on GitHub Actions

Extend the existing CI workflow with an ubuntu-only "End-to-end (soulfind)"
job that builds soulfind from source (LDC + `dub build :server`), points
SOULFIND_BIN at it, and runs the e2e suite with SOULSEEK_E2E_REQUIRED=1.
Also widen the clippy job to the whole workspace.

The e2e harness gains SOULSEEK_E2E_REQUIRED: when set, a missing/unstartable
server is a hard failure instead of a silent skip, so CI genuinely exercises
the client end-to-end. Add e2e coverage for two existing behaviours a single
server can verify: wrong-password rejection and concurrent logins.

soulseek-rs/src/main.rs

@@ -56,14 +56,15 @@ fn main() -> Result<()> {
 
     // Resolve the optional shared/upload directory up front; a misconfigured
     // one is a warning, not a fatal error (the client just shares nothing).
-    let shared_directory =
-        match directories::resolve_shared_directory(cli.shared_dir.as_deref()) {
-            Ok(dir) => dir.map(|path| path.display().to_string()),
-            Err(e) => {
-                eprintln!("⚠️  Ignoring shared directory: {e}");
-                None
-            }
-        };
+    let shared_directory = match directories::resolve_shared_directory(
+        cli.shared_dir.as_deref(),
+    ) {
+        Ok(dir) => dir.map(|path| path.display().to_string()),
+        Err(e) => {
+            eprintln!("⚠️  Ignoring shared directory: {e}");
+            None
+        }
+    };
 
     match cli.command {
         Some(Commands::Search {