feat: add VitePress docs site, music file picker modal, Dockerfile optimization, i18n keys for 14 locales

- Add docs/ with VitePress site (19 pages): guide, config, architecture, FAQ
- Add GitHub Actions workflow for auto-deploy to GitHub Pages
- Replace music 'Add Tracks' upload picker with in-app audio file browser modal
- Add music picker CSS styles with dark theme support
- Add missing i18n keys (search_audio, no_audio_files, etc.) to all 14 locales
- Optimize Dockerfile: shared base stage, COPY --chmod, consolidated RUN, HEALTHCHECK
- Improve README: docs links, updated stats (222+ tests, 14 languages), feature status

docs/public/logo.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 500 500">
+  <!-- Fondo circular -->
+  <circle cx="250" cy="250" r="200" fill="#f8f9fa" />
+  
+  <!-- Forma de nube principal - inspirada en Nextcloud pero con estilo propio -->
+  <path d="M330 280c27.6 0 50-22.4 50-50s-22.4-50-50-50c-5.3 0-10.3 0.8-15.1 2.3C307.4 154.5 282.6 135 253 135c-29.6 0-54.4 19.5-62.9 46.3C185.3 180.8 180.3 180 175 180c-27.6 0-50 22.4-50 50s22.4 50 50 50h155z" fill="#ff5e3a" />
+  
+  <!-- Texto del logotipo - solo el nombre -->
+  <text x="250" y="350" font-family="Arial, sans-serif" font-size="55" font-weight="bold" text-anchor="middle" fill="#2b3a4a">OxiCloud</text>
+</svg>