feat(server,web): expose server version via /healthz + display in Settings

Came up debugging the in-app update flow — wasn't obvious which image
the container was actually running without exec'ing in. New flow:

### Server
- internal/server/version.go: new `var ServerVersion = "dev"`,
  overridden via -ldflags at build time.
- /healthz response gains a "version" key alongside the existing
  "status" + "min_client_version". Backward-compat: existing clients
  ignore unknown JSON fields. Endpoint stays unauthenticated.

### Build
- Dockerfile: new `ARG MINSTREL_VERSION=dev`, threaded into the
  go build -ldflags so the binary's ServerVersion is stamped at
  link time. Default "dev" preserves local `docker build` ergonomics.
- .forgejo/workflows/release.yml: tags step also emits a `version`
  output (the git tag for tag pushes, "main" for branch pushes);
  build step passes it as `--build-arg MINSTREL_VERSION=...`.

### Web
- web/src/lib/components/ServerVersion.svelte: small understated
  text ("Server v2026.05.10.2") that fetches /healthz on mount.
  Renders nothing on parse failure or pre-version images.
- Mounted at the bottom of Settings.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.forgejo/workflows/release.yml

@@ -50,9 +50,11 @@ jobs:
           if [[ "${GITHUB_REF}" == refs/tags/v* ]]; then
             VERSION="${GITHUB_REF#refs/tags/}"
             echo "args=-t ${IMAGE}:${VERSION} -t ${IMAGE}:latest" >> "$GITHUB_OUTPUT"
+            echo "version=${VERSION}" >> "$GITHUB_OUTPUT"
             echo "::notice::Release build: ${VERSION} + latest"
           else
             echo "args=-t ${IMAGE}:main" >> "$GITHUB_OUTPUT"
+            echo "version=main" >> "$GITHUB_OUTPUT"
             echo "::notice::Main-branch build: :main"
           fi
 
@@ -93,4 +95,7 @@ jobs:
 
       - name: Build and push
         if: steps.guard.outputs.ready == 'true'
-        run: docker buildx build --push ${{ steps.tags.outputs.args }} .
+        run: |
+          docker buildx build \
+            --build-arg MINSTREL_VERSION="${{ steps.tags.outputs.version }}" \
+            --push ${{ steps.tags.outputs.args }} .