fix(status): stop false LOAD FAILED from a relayed adsb.lol 429 and the installations zoom prompt

Two independent triggers turned the global status chip red while every layer
was still populated (reproduced at JFK in Cockpit; timeline in the issue).

adsb.lol proxy: a non-OK upstream status was relayed verbatim even with a
12 s-old cache in hand, and the browser layer answers a 429 with a 45 s
cooldown in which every refresh reports "adsb.lol rate limited" without a
network call. The proxy now serves its cached body as STALE on a 429/5xx and
backs off from upstream for Retry-After (bounded), mirroring the OpenSky
proxy. A failure with nothing cached is still relayed.

Mapped installations: the "zoom in" guidance was stored in `error`, so the
manager reported a refresh failure and the chip a LOAD FAILED whenever the
view could not be bounded (Cockpit looks at the horizon). The status is now
set without an error string; the row reads the prompt from `statusMessage`,
and the chip ignores `error` for guidance statuses, matching the carve-out
layerFeedState already has.

src/data/manager.test.mjs

@@ -3256,3 +3256,14 @@ test('a layer that surrenders its row controls hides the block entirely', async
     else globalThis.document = originalDocument;
   }
 });
+
+test('layer metadata shows a guidance prompt without reporting it as a fault', () => {
+  const mgr = new DataLayerManager({});
+  const text = mgr._buildMetaText({
+    enabled: true,
+    source: 'OpenStreetMap',
+    stats: { status: 'zoom-in', error: null, count: 0, statusMessage: 'Zoom in to search mapped installations' },
+  });
+  assert.match(text, /Zoom in to search mapped installations$/);
+  assert.doesNotMatch(text, /UNAVAILABLE|DEGRADED/);
+});