refactor: replace chromedp with browser native print for PDF export

Remove chromedp dependency, use window.print() instead. Docker image no longer needs Chromium (~200MB smaller).

- Delete services/pdf.go and templates/reports/review.html

- Remove PDF API route POST /api/repos/:id/review/pdf

- Add @media print CSS to review page

- Remove chromium from Dockerfile

Dockerfile

@@ -9,8 +9,7 @@ RUN CGO_ENABLED=1 go build -o pr-helper .
 
 # Runtime stage
 FROM alpine:3.20
-RUN apk add --no-cache ca-certificates chromium
-ENV CHROME_BIN=/usr/bin/chromium-browser
+RUN apk add --no-cache ca-certificates
 WORKDIR /app
 COPY --from=builder /app/pr-helper .
 COPY --from=builder /app/templates ./templates