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/routes/+layout.svelte

@@ -0,0 +1,39 @@
+<script>
+  let { children } = $props();
+</script>
+
+{@render children()}
+
+<style>
+  :global(html, body) {
+    margin: 0;
+    padding: 0;
+    background: #0a0a23;
+    color: #e0e0e0;
+    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen,
+      Ubuntu, Cantarell, sans-serif;
+    overflow: hidden;
+    height: 100%;
+  }
+
+  :global(*, *::before, *::after) {
+    box-sizing: border-box;
+  }
+
+  :global(::-webkit-scrollbar) {
+    width: 8px;
+  }
+
+  :global(::-webkit-scrollbar-track) {
+    background: #0a0a23;
+  }
+
+  :global(::-webkit-scrollbar-thumb) {
+    background: #2a3a5e;
+    border-radius: 4px;
+  }
+
+  :global(::-webkit-scrollbar-thumb:hover) {
+    background: #4a5568;
+  }
+</style>