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>

web/src/routes/settings/+page.svelte

@@ -18,6 +18,7 @@
   import { errCode } from '$lib/api/errors';
   import { pushToast } from '$lib/stores/toast.svelte';
   import MobileAppDownload from '$lib/components/MobileAppDownload.svelte';
+  import ServerVersion from '$lib/components/ServerVersion.svelte';
 
   const queryClient = useQueryClient();
 
@@ -326,4 +327,8 @@
   </section>
 
   <MobileAppDownload />
+
+  <div class="pt-2 text-center">
+    <ServerVersion />
+  </div>
 </div>