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/+page.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { invoke } from '@tauri-apps/api/core';
+  import { invoke, convertFileSrc } from '@tauri-apps/api/core';
   import { open, save } from '@tauri-apps/plugin-dialog';
   import WaveformPlayer from '$lib/components/WaveformPlayer.svelte';
   import TranscriptEditor from '$lib/components/TranscriptEditor.svelte';
@@ -81,8 +81,8 @@
     });
     if (!filePath) return;
 
-    // Convert file path to URL for wavesurfer
-    audioUrl = `asset://localhost/${encodeURIComponent(filePath)}`;
+    // Convert file path to asset URL for wavesurfer
+    audioUrl = convertFileSrc(filePath);
     waveformPlayer?.loadAudio(audioUrl);
 
     // Start pipeline (transcription + diarization)