feat v1.42.0: room settings — the third settings tier

- four-tier principle fixed in ARCHITECTURE: global > space > room >
  device, specific overrides general, unset inherits
- room.settings { fill_mode, temp_source, hum_source } with backend
  schema; pure roomFillModeOf + sourceValue (+2 test suites, 114)
- gear on room cards in the Plan editor opens Room settings: rename,
  re-area (current area included in the list), fill override (may opt
  out of glow darkness), explicit temp/hum source with a searchable
  device+entity dropdown; the same section in the creation dialog
- source feeds the room card, tooltip and temp fill; works for rooms
  without an HA area (user-feedback case #1)
- smoke_room_settings.mjs (10 checks); TESTING/CHANGELOG/ARCHITECTURE
  same-commit

src/styles.ts

@@ -378,6 +378,15 @@ export const cardStyles = css`
       text-align: center;
     }
     .rlname { display: inline-flex; align-items: center; gap: 0.25em; }
+    .rlgear {
+      --mdc-icon-size: 0.9em;
+      display: inline-flex;
+      margin-right: 0.2em;
+      opacity: 0.6;
+      cursor: pointer;
+      pointer-events: auto;
+    }
+    .rlgear:hover { opacity: 1; }
     .rlgo {
       --mdc-icon-size: 0.85em;
       display: inline-flex;