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).

Dockerfile.android

@@ -34,20 +34,20 @@ FROM tools AS builder
 WORKDIR /app
 
 # Abhängigkeiten cachen
-COPY client/package.json client/bun.lock* ./client/
-RUN cd client && bun install
+COPY package.json bun.lock* ./
+RUN bun install
 
 COPY . .
 
 # 1. Web-App bauen (ohne PWA Service Worker für Capacitor)
-RUN cd client && CAPACITOR=1 bun run build
+RUN CAPACITOR=1 bun run build
 
 # 2. Web-Assets in das Android-Projekt synchronisieren
 #    (android/ ist im Repo eingecheckt – kein cap add nötig)
-RUN cd client && CI=true ./node_modules/.bin/cap sync android
+RUN CI=true ./node_modules/.bin/cap sync android
 
 # 3. APK bauen
-RUN cd client/android && \
+RUN cd android && \
     chmod +x gradlew && \
     GRADLE_OPTS="-Xmx3g -XX:MaxMetaspaceSize=512m" \
     ./gradlew assembleDebug --no-daemon
@@ -55,5 +55,5 @@ RUN cd client/android && \
 # ── Stage 3: Nur die APK (für --output) ──────────────────────────────────────
 FROM scratch
 COPY --from=builder \
-    /app/client/android/app/build/outputs/apk/debug/app-debug.apk \
+    /app/android/app/build/outputs/apk/debug/app-debug.apk \
     /tiptoi-sync.apk