SMB Server Phase 2: VFS backend build fix + integration test

- Add VfsFile: Send supertrait for Mutex compatibility
- Fix SmbServerCommand: struct → Subcommand enum with Start variant
- Fix tracing_subscriber::init() → try_init() to avoid panic when
  logger already initialized
- Fix CLI subcommand name: smb-server → smb-start (flatten naming)
- Add #[command(name = "smb-start")] for CLI disambiguation
- Fix unused variable warnings (smb_fs.rs, smb_server_backend.rs)
- Remove unused VfsFile imports (webdav.rs, scp_handler.rs)
- Integration test: Docker smbclient verified (list, upload, read)

markbase-core/src/cli/tools/mod.rs

@@ -1,4 +1,5 @@
 pub mod render;
+pub mod smb_server;
 pub mod test;
 
 use clap::Subcommand;
@@ -9,12 +10,15 @@ pub enum ToolsCommands {
     Render(render::RenderCommand),
     #[command(flatten)]
     Test(test::TestCommand),
+    #[command(flatten)]
+    SmbServer(smb_server::SmbServerCommand),
 }
 
 pub async fn handle_tools_command(cmd: ToolsCommands) -> anyhow::Result<()> {
     match cmd {
         ToolsCommands::Render(c) => render::handle_render_command(c)?,
         ToolsCommands::Test(c) => test::handle_test_command(c)?,
+        ToolsCommands::SmbServer(c) => smb_server::handle_smb_server_command(c).await?,
     }
     Ok(())
 }