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/dashboard.css

@@ -434,28 +434,8 @@ body.dashboard-shell {
 }
 .port-conflict strong { color: var(--red); }
 
-.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;
-}
+/* .notif-row / .btn-notif moved to settings.css with the S3TT1NGS
+   page. */
 
 .pending-state {
   color: var(--amber);