Add bash shell tab and file manager for running containers

Adds two new features for running project containers:

1. Bash Shell Tab: A "Shell" button on running projects opens a plain
   bash -l session instead of Claude Code, useful for direct container
   inspection, package installation, and debugging. Tab labels show
   "(bash)" suffix to distinguish from Claude sessions.

2. File Manager: A "Files" button opens a modal file browser for
   navigating container directories, downloading files to the host,
   and uploading files from the host. Supports breadcrumb navigation
   and works with any path including those outside mounted projects.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src-tauri/src/docker/exec.rs

@@ -277,3 +277,41 @@ impl ExecSessionManager {
         Ok(format!("/tmp/{}", file_name))
     }
 }
+
+/// Run a one-shot (non-interactive) exec command in a container and collect stdout.
+pub async fn exec_oneshot(container_id: &str, cmd: Vec<String>) -> Result<String, String> {
+    let docker = get_docker()?;
+
+    let exec = docker
+        .create_exec(
+            container_id,
+            CreateExecOptions {
+                attach_stdout: Some(true),
+                attach_stderr: Some(true),
+                cmd: Some(cmd),
+                user: Some("claude".to_string()),
+                ..Default::default()
+            },
+        )
+        .await
+        .map_err(|e| format!("Failed to create exec: {}", e))?;
+
+    let result = docker
+        .start_exec(&exec.id, None)
+        .await
+        .map_err(|e| format!("Failed to start exec: {}", e))?;
+
+    match result {
+        StartExecResults::Attached { mut output, .. } => {
+            let mut stdout = String::new();
+            while let Some(msg) = output.next().await {
+                match msg {
+                    Ok(data) => stdout.push_str(&String::from_utf8_lossy(&data.into_bytes())),
+                    Err(e) => return Err(format!("Exec output error: {}", e)),
+                }
+            }
+            Ok(stdout)
+        }
+        StartExecResults::Detached => Err("Exec started in detached mode".to_string()),
+    }
+}