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/ai.rs

@@ -1,19 +1,7 @@
 use serde_json::{json, Value};
 
 use crate::sidecar::messages::IPCMessage;
-use crate::sidecar::SidecarManager;
-
-fn get_sidecar() -> Result<SidecarManager, 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 manager = SidecarManager::new();
-    manager.start(&python_path.to_string_lossy())?;
-    Ok(manager)
-}
+use crate::sidecar::sidecar;
 
 /// Send a chat message to the AI provider via the Python sidecar.
 #[tauri::command]
@@ -22,14 +10,8 @@ pub fn ai_chat(
     transcript_context: Option<String>,
     provider: Option<String>,
 ) -> Result<Value, String> {
-    let manager = get_sidecar()?;
-
-    let request_id = uuid::Uuid::new_v4().to_string();
-    let payload = json!({
-        "action": "chat",
-        "messages": messages,
-        "transcript_context": transcript_context.unwrap_or_default(),
-    });
+    let manager = sidecar();
+    manager.ensure_running()?;
 
     // If a specific provider is requested, set it first
     if let Some(p) = provider {
@@ -41,7 +23,17 @@ pub fn ai_chat(
         let _ = manager.send_and_receive(&set_msg)?;
     }
 
-    let msg = IPCMessage::new(&request_id, "ai.chat", payload);
+    let request_id = uuid::Uuid::new_v4().to_string();
+    let msg = IPCMessage::new(
+        &request_id,
+        "ai.chat",
+        json!({
+            "action": "chat",
+            "messages": messages,
+            "transcript_context": transcript_context.unwrap_or_default(),
+        }),
+    );
+
     let response = manager.send_and_receive(&msg)?;
 
     if response.msg_type == "error" {
@@ -57,7 +49,8 @@ pub fn ai_chat(
 /// List available AI providers.
 #[tauri::command]
 pub fn ai_list_providers() -> Result<Value, String> {
-    let manager = get_sidecar()?;
+    let manager = sidecar();
+    manager.ensure_running()?;
 
     let request_id = uuid::Uuid::new_v4().to_string();
     let msg = IPCMessage::new(
@@ -73,7 +66,8 @@ pub fn ai_list_providers() -> Result<Value, String> {
 /// Configure an AI provider with API key/settings.
 #[tauri::command]
 pub fn ai_configure(provider: String, config: Value) -> Result<Value, String> {
-    let manager = get_sidecar()?;
+    let manager = sidecar();
+    manager.ensure_running()?;
 
     let request_id = uuid::Uuid::new_v4().to_string();
     let msg = IPCMessage::new(