Add CalVer build-time version tracking

- Dockerfile: ARG BUILD_VERSION=dev → ENV APP_VERSION baked into image
- ci.yml: BUILD_VERSION passed as build-arg; set to git tag name on v*
  tag builds, "dev" on branch builds
- routes/api.py: GET /api/version returns {"version": APP_VERSION}
- SettingsView: fetches /api/version on mount, displays in About section
  under General tab

Version source of truth is the git tag (YY.MM.DD.N CalVer).
pyproject.toml / package.json versions are no longer maintained.

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

src/fabledassistant/routes/api.py

@@ -1,3 +1,5 @@
+import os
+
 from quart import Blueprint, jsonify
 
 api = Blueprint("api", __name__, url_prefix="/api")
@@ -6,3 +8,8 @@ api = Blueprint("api", __name__, url_prefix="/api")
 @api.route("/health")
 async def health():
     return jsonify({"status": "ok"})
+
+
+@api.route("/version")
+async def version():
+    return jsonify({"version": os.environ.get("APP_VERSION", "dev")})