fix: remove go.sum from Dockerfile COPY since project has no external dependencies

The project uses only Go stdlib with zero external dependencies, so go.sum
does not exist. The Dockerfile COPY instruction fails when go.sum is missing.

Closes leeworks-agents/gitea-mobile#89

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -1,7 +1,7 @@
 # Stage 1: Build
 FROM golang:1.22-alpine AS builder
 WORKDIR /app
-COPY go.mod go.sum ./
+COPY go.mod ./
 RUN go mod download
 COPY . .
 RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /gitea-mobile ./cmd/server