Integrate whynot-design into Economic Observatory UI

Vendor whynot-design Layer 1 (tokens, CSS) and Layer 2 (<wn-*>
components) via scripts/sync-whynot-design.sh with a pinned ref.
Migrate the observatory shell to canonical web components, keep
observatory-specific layout in styles.css, and add vendor integrity
tests plus correct JS MIME types on the dev server.

projects/coulomb-pricing/observatory/server.py

@@ -11,6 +11,13 @@ from .api import build_dashboard_payload
 ROOT = Path(__file__).resolve().parent.parent
 UI_DIR = ROOT / "ui"
 
+UI_CONTENT_TYPES = {
+    ".css": "text/css",
+    ".js": "application/javascript",
+    ".json": "application/json",
+    ".html": "text/html; charset=utf-8",
+}
+
 
 class ObservatoryHandler(BaseHTTPRequestHandler):
     data_dir: Path = ROOT / "data"
@@ -38,8 +45,10 @@ class ObservatoryHandler(BaseHTTPRequestHandler):
             relative = parsed.path.removeprefix("/ui/")
             target = UI_DIR / relative
             if target.exists() and target.is_file():
-                content_type = "text/css" if target.suffix == ".css" else "application/javascript"
-                return self._serve_file(target, f"{content_type}; charset=utf-8")
+                content_type = UI_CONTENT_TYPES.get(target.suffix, "application/octet-stream")
+                if "charset" not in content_type:
+                    content_type = f"{content_type}; charset=utf-8"
+                return self._serve_file(target, content_type)
 
         self._send(404, b"Not found", "text/plain")