Fix critical integration issues for end-to-end functionality

- Rewrite SidecarManager as singleton with OnceLock, reusing one Python
  process across all commands instead of spawning per call
- Separate stdin/stdout ownership with dedicated BufReader to prevent
  data corruption between wait_for_ready and send_and_receive
- Add ensure_running() for auto-start on first command
- Fix asset protocol URL: use convertFileSrc() instead of manual
  encodeURIComponent which broke file paths with slashes
- Add +layout.svelte with global dark theme, CSS reset, and custom
  scrollbar styling to prevent white flash on startup
- Register AppState with Tauri .manage(), initialize SQLite database
  on app startup at ~/.voicetonotes/voice_to_notes.db
- Wire project commands (create/get/list) to real database queries
  instead of placeholder stubs

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

src-tauri/src/commands/transcribe.rs

@@ -1,12 +1,9 @@
 use serde_json::{json, Value};
 
 use crate::sidecar::messages::IPCMessage;
-use crate::sidecar::SidecarManager;
+use crate::sidecar::sidecar;
 
 /// Start transcription of an audio file via the Python sidecar.
-///
-/// This is a blocking command — it starts the sidecar if needed,
-/// sends the transcribe request, and waits for the result.
 #[tauri::command]
 pub fn transcribe_file(
     file_path: String,
@@ -14,17 +11,8 @@ pub fn transcribe_file(
     device: Option<String>,
     language: Option<String>,
 ) -> Result<Value, String> {
-    // Determine Python sidecar path (relative to app)
-    let python_path = std::env::current_dir()
-        .map_err(|e| e.to_string())?
-        .join("../python")
-        .canonicalize()
-        .map_err(|e| format!("Cannot find python directory: {e}"))?;
-
-    let python_path_str = python_path.to_string_lossy().to_string();
-
-    let manager = SidecarManager::new();
-    manager.start(&python_path_str)?;
+    let manager = sidecar();
+    manager.ensure_running()?;
 
     let request_id = uuid::Uuid::new_v4().to_string();
     let msg = IPCMessage::new(
@@ -63,16 +51,8 @@ pub fn run_pipeline(
     max_speakers: Option<u32>,
     skip_diarization: Option<bool>,
 ) -> Result<Value, String> {
-    let python_path = std::env::current_dir()
-        .map_err(|e| e.to_string())?
-        .join("../python")
-        .canonicalize()
-        .map_err(|e| format!("Cannot find python directory: {e}"))?;
-
-    let python_path_str = python_path.to_string_lossy().to_string();
-
-    let manager = SidecarManager::new();
-    manager.start(&python_path_str)?;
+    let manager = sidecar();
+    manager.ensure_running()?;
 
     let request_id = uuid::Uuid::new_v4().to_string();
     let msg = IPCMessage::new(