Add exercise numbers, image uploads, version display, session resume, and training sparklines

- Exercise number (UF#): optional field on exercises, displayed in cards, training, and sets
- Import training plan numbers via migration 005 (UPDATE by name)
- Exercise images: JPG upload with multi-image support per exercise (migration 006)
- Version endpoint (GET /api/v1/version) with ldflags injection in Makefile and Dockerfile
- Version displayed on settings page
- Session resume: GET /api/v1/sessions/active endpoint, auto-resume on training page load
- Block new session while one is active (409 Conflict)
- e1RM sparkline chart per exercise during training (Epley formula)
- Fix CORS: add X-User-ID to allowed headers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -15,7 +15,8 @@ RUN go mod download
 COPY backend/ ./
 # Frontend-Build in static/ einhängen (wird per embed eingebettet)
 COPY --from=frontend-builder /app/frontend/dist/ ./static/
-RUN CGO_ENABLED=1 go build -ldflags="-s -w" -o krafttrainer ./cmd/server
+ARG VERSION=dev
+RUN CGO_ENABLED=1 go build -ldflags="-s -w -X main.Version=${VERSION}" -o krafttrainer ./cmd/server
 
 # Stage 3: Minimales Runtime-Image
 FROM debian:bookworm-slim