Fix sidecar stdout buffering: set PYTHONUNBUFFERED=1

PyInstaller frozen executables buffer stdout when piped to a
subprocess (no TTY). Even with flush=True in Python, the OS-level
pipe buffer can delay output. This prevented the ready event from
reaching the Tauri app, causing the "Starting sidecar..." hang.

Fix: set PYTHONUNBUFFERED=1 env var on both prod and dev sidecar
commands, plus -u flag for dev mode Python.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/sidecar/mod.rs

@@ -555,7 +555,7 @@ impl SidecarManager {
 
     fn build_dev_command(&self) -> Result<std::process::Command, String> {
         let mut cmd = std::process::Command::new("python");
-        cmd.args(["-m", "backend.main_headless"]);
+        cmd.args(["-u", "-m", "backend.main_headless"]); // -u = unbuffered
 
         // Try to find the project root (parent of src-tauri)
         if let Some(dirs) = DIRS.get() {
@@ -568,6 +568,7 @@ impl SidecarManager {
             }
         }
 
+        cmd.env("PYTHONUNBUFFERED", "1");
         Ok(cmd)
     }
 
@@ -583,6 +584,9 @@ impl SidecarManager {
             bin.parent()
                 .ok_or("Cannot determine sidecar parent dir")?,
         );
+        // Force unbuffered stdout so the ready event is sent immediately.
+        // PyInstaller frozen executables buffer stdout when piped.
+        cmd.env("PYTHONUNBUFFERED", "1");
         Ok(cmd)
     }