fix: remove conflicting middleware.ts, proxy.ts already handles locale routing

Next.js 16 uses proxy.ts instead of middleware.ts. The existing proxy.ts
already includes next-intl locale routing + auth guards. CACHE_BUST=5.

Dockerfile

@@ -4,7 +4,7 @@ WORKDIR /app
 
 # --- Install + Build in one stage ---
 FROM base AS builder
-ARG CACHE_BUST=4
+ARG CACHE_BUST=5
 COPY . .
 RUN pnpm install --no-frozen-lockfile
 ENV NEXT_TELEMETRY_DISABLED=1