refactor: restructure project to standard Go layout

- Reorganize code into cmd/bilidown and internal/ packages
- Rename client/ to web/ for frontend source
- Remove systray dependency for headless web service
- Embed static files into binary using go:embed
- Update import paths to use internal/ prefix
- Update .gitignore with common patterns

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

web/vite.config.ts

@@ -0,0 +1,22 @@
+import { defineConfig } from 'vite'
+
+export default defineConfig({
+    clearScreen: false,
+    server: {
+        proxy: {
+            '/api': 'http://127.0.0.1:8098'
+        },
+        host: '0.0.0.0'
+    },
+    build: {
+        outDir: '../build/static',
+        emptyOutDir: true,
+    },
+    css: {
+        preprocessorOptions: {
+            scss: {
+                api: 'modern-compiler'
+            }
+        }
+    }
+})