fix(build): add -tags embed to ensure frontend is embedded

- Add -tags=embed flag to GoReleaser builds
- Add -tags embed flag to Dockerfile builds
- Fix Dockerfile COPY order to prevent frontend dist being overwritten
- Update README build instructions with embed tag explanation

Dockerfile

@@ -40,14 +40,15 @@ WORKDIR /app/backend
 COPY backend/go.mod backend/go.sum ./
 RUN go mod download
 
-# Copy frontend dist from previous stage
-COPY --from=frontend-builder /app/frontend/../backend/internal/web/dist ./internal/web/dist
-
-# Copy backend source
+# Copy backend source first
 COPY backend/ ./
 
-# Build the binary (BuildType=release for CI builds)
+# Copy frontend dist from previous stage (must be after backend copy to avoid being overwritten)
+COPY --from=frontend-builder /app/backend/internal/web/dist ./internal/web/dist
+
+# Build the binary (BuildType=release for CI builds, embed frontend)
 RUN CGO_ENABLED=0 GOOS=linux go build \
+    -tags embed \
     -ldflags="-s -w -X main.Commit=${COMMIT} -X main.Date=${DATE:-$(date -u +%Y-%m-%dT%H:%M:%SZ)} -X main.BuildType=release" \
     -o /app/sub2api \
     ./cmd/server