feat(web): restyle Settings + read-only library path panel (Phase 3)

Settings onto the Pressing Plant system: credentials with clear set/unset
affordances, a Library section that shows the configured /music path with a
"change MUSIC_DIR in docker-compose + rebuild" note (honest about the mount
being non-remountable from the UI), restyled scan trigger/result, and the
discovery section. All logic and aria-labels preserved. Completes the
frontend redesign (all pages + shell on one system, both themes).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/design.css

@@ -222,3 +222,23 @@ nav.contents .sep { flex: 1; }
 }
 .tracks li .tnum { color: var(--rule-2); min-width: 2em; }
 .tracks li .tname { color: var(--ink); }
+
+/* ── Phase 3: settings ────────────────────────────────── */
+.field-grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(240px, 1fr)); gap: 18px 24px; margin: 16px 0 8px; }
+.field.wide { grid-column: 1 / -1; }
+.subhead {
+  font-family: var(--mono); font-size: 0.66rem; letter-spacing: 0.16em; text-transform: uppercase;
+  color: var(--graphite); margin: 26px 0 2px;
+}
+.field .set { color: var(--accent); }
+.info-panel {
+  background: var(--paper-2); border: 1px solid var(--rule); padding: 15px 18px; margin: 14px 0 16px;
+  font-family: var(--mono); font-size: 0.78rem; line-height: 1.8; color: var(--ink);
+}
+.info-panel .k { color: var(--graphite); text-transform: uppercase; letter-spacing: 0.12em; font-size: 0.6rem; }
+.info-panel code { color: var(--accent); }
+.saved-note {
+  font-family: var(--mono); font-size: 0.64rem; letter-spacing: 0.12em; text-transform: uppercase; color: var(--accent);
+}
+.save-row { display: flex; align-items: center; gap: 14px; margin-top: 8px; }
+.settings-section { margin-bottom: 30px; }