Flatten repo: move client/ contents to repo root

The project is no longer a monorepo/workspace - there is only a single
app package. Remove the redundant client/ nesting: all source, the
Capacitor Android project, config files and package.json/tsconfig now
live directly at the repo root.

- Merge client/package.json into root package.json (drop the
  workspaces field and the now-unnecessary bun --cwd client wrappers).
- Merge client/bun.lock (the actual dependency lockfile) into the
  root, replacing the stale workspace-only root lockfile.
- Move src/, public/, android/, index.html, vite.config.ts,
  capacitor.config.ts, tsconfig.{app,node}.json to the repo root.
- Update Dockerfile.android to build from the repo root instead of
  cd'ing into client/.
- Update .github/workflows/ci.yml type-check step to run bun run build
  directly instead of bun --cwd client run build.
- Update README.md and .github/copilot-instructions.md to drop
  references to the client/ subdirectory.
- Drop the stale server/downloads/ entry from .gitignore (leftover
  from an old server-based architecture that no longer exists).

capacitor.config.ts

@@ -0,0 +1,19 @@
+import type { CapacitorConfig } from '@capacitor/cli'
+
+// DEV_URL aktiviert den Capacitor Live-Reload gegen den Vite-Dev-Server.
+// Beispiel: DEV_URL=http://192.168.1.42:5173 bun run cap:android
+const devUrl = process.env.DEV_URL
+
+const config: CapacitorConfig = {
+  appId: 'com.tiptoisync.app',
+  appName: 'tiptoi Sync',
+  webDir: 'dist',
+  server: devUrl
+    ? { url: devUrl, cleartext: true }
+    : { androidScheme: 'https' },
+  android: {
+    allowMixedContent: true,
+  },
+}
+
+export default config