refactor(frontend): move S3TT1NGS to its own /settings.html page (#1464 step 2)

Extracts the operator-local preferences (browser-notification toggle)
out of the dashboard tab strip into a standalone /settings.html page,
reached from the H0M3 hub — same minimal-chrome pattern as /flow.html
and /logs.html. The dashboard tab strip is now purely operational.

- new settings.{html,css,js}; settings.js binds NOTIF + renders the
  shared server-warnings banner (matching the other standalone pages).
- drop the S3TT1NGS tab + pane from dashboard.html; the NOTIF.bind()
  toggle wiring moves to settings.js. The dashboard keeps NOTIF.show()
  (approvals / questions) — it reads the same browser permission +
  localStorage mute flag the settings page sets, so firing still works.
- move .notif-row / .btn-notif from dashboard.css to settings.css.
- add a Settings tile to the H0M3 hub; wire the new entries into
  build.mjs and update the nix/frontend.nix asset manifest comment.

First slice of #1464 step 2; the ST4TS page extraction follows separately.

frontend/packages/dashboard/src/settings.css

@@ -0,0 +1,40 @@
+/* ─── /settings.html — operator-local preferences ──────────────────
+   Extracted from the dashboard S3TT1NGS tab. Same
+   minimal-chrome pattern as /flow.html and /logs.html: the back-link
+   header + title (.logs-header / .logs-back / .logs-title) live in
+   common.css; only the page body padding + the notif-toggle row are
+   page-specific and live here. */
+
+body.settings-shell {
+  margin: 0;
+  padding: 0;
+}
+
+.settings-main {
+  padding: 1.2em 1.5em 2em;
+}
+
+/* Browser-notification toggle row (moved verbatim from dashboard.css —
+   the buttons + status that NOTIF.bind() in common.js wires up). */
+.notif-row {
+  display: flex;
+  gap: 0.5em;
+  align-items: center;
+  margin: 0.5em 0;
+  font-size: 0.85em;
+}
+.btn-notif {
+  font-family: inherit;
+  font-size: 0.85em;
+  background: transparent;
+  color: var(--cyan);
+  border: 1px solid var(--cyan);
+  padding: 0.2em 0.7em;
+  border-radius: 999px;
+  cursor: pointer;
+  text-shadow: 0 0 4px currentColor;
+}
+.btn-notif:hover {
+  background: color-mix(in srgb, var(--cyan) 10%, transparent);
+  box-shadow: 0 0 10px -2px currentColor;
+}