Fix IPC stdout corruption, dark window background, overlay timing

- Redirect sys.stdout to stderr in Python sidecar so library print()
  calls don't corrupt the JSON-line IPC stream
- Save real stdout fd for exclusive IPC use via init_ipc()
- Skip non-JSON lines in Rust reader instead of failing with parse error
- Set Tauri window background color to match dark theme (#0a0a23)
- Add inline dark background on html/body to prevent white flash
- Use Svelte tick() to ensure progress overlay renders before invoke
- Improve ProgressOverlay with spinner, better styling, z-index 9999

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

src-tauri/src/sidecar/mod.rs

@@ -107,8 +107,9 @@ impl SidecarManager {
                         return Ok(());
                     }
                 }
-                // Non-ready message: something is wrong
-                break;
+                // Non-JSON or non-ready line — skip and keep waiting
+                eprintln!("[sidecar-rs] Skipping pre-ready line: {}", &trimmed[..trimmed.len().min(200)]);
+                continue;
             }
         }
         Err("Sidecar did not send ready message".to_string())
@@ -160,8 +161,14 @@ impl SidecarManager {
                     if trimmed.is_empty() {
                         continue;
                     }
-                    let response: IPCMessage = serde_json::from_str(trimmed)
-                        .map_err(|e| format!("Parse error: {e}"))?;
+                    // Skip non-JSON lines (library output that leaked to stdout)
+                    let response: IPCMessage = match serde_json::from_str(trimmed) {
+                        Ok(msg) => msg,
+                        Err(_) => {
+                            eprintln!("[sidecar-rs] Skipping non-JSON line: {}", &trimmed[..trimmed.len().min(200)]);
+                            continue;
+                        }
+                    };
 
                     if response.msg_type == "progress" {
                         on_progress(&response);