Fix dev mode sidecar launch and engine reload on mode change

1. Dev mode: use `uv run python` instead of bare `python` to ensure
   the project venv is used. Also use CARGO_MANIFEST_DIR to find the
   project root reliably.

2. Engine reload: changing remote.mode (local/managed/byok) now
   triggers a full engine reload. Previously only model and device
   changes triggered reload, so switching to Deepgram had no effect
   until the app was restarted.

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

src-tauri/src/sidecar/mod.rs

@@ -554,18 +554,27 @@ impl SidecarManager {
     // -- private helpers -------------------------------------------------------
 
     fn build_dev_command(&self) -> Result<std::process::Command, String> {
-        let mut cmd = std::process::Command::new("python");
-        cmd.args(["-u", "-m", "backend.main_headless"]); // -u = unbuffered
+        // Use `uv run` to ensure we use the project's venv, not system Python
+        let mut cmd = std::process::Command::new("uv");
+        cmd.args(["run", "python", "-u", "-m", "backend.main_headless"]);
 
-        // Try to find the project root (parent of src-tauri)
-        if let Some(dirs) = DIRS.get() {
-            let project_root = dirs
-                .resource_dir
-                .parent() // src-tauri
-                .and_then(|p| p.parent()); // project root
-            if let Some(root) = project_root {
-                cmd.current_dir(root);
-            }
+        // Find the project root: try CARGO_MANIFEST_DIR first (set at compile time),
+        // then fall back to resource_dir parent chain
+        let manifest_dir = option_env!("CARGO_MANIFEST_DIR").map(std::path::PathBuf::from);
+        let project_root = manifest_dir
+            .as_ref()
+            .and_then(|d| d.parent()) // src-tauri -> project root
+            .or_else(|| {
+                DIRS.get()
+                    .and_then(|d| d.resource_dir.parent())
+                    .and_then(|p| p.parent())
+            });
+
+        if let Some(root) = project_root {
+            eprintln!("[sidecar] Dev mode: working dir = {}", root.display());
+            cmd.current_dir(root);
+        } else {
+            eprintln!("[sidecar] Dev mode: WARNING - could not determine project root");
         }
 
         cmd.env("PYTHONUNBUFFERED", "1");