feat: homeserver status banner from Uptime Kuma (#124)

When the homeserver, voice calls or sign-in break, or maintenance is under
way, say so in the app — driven by the Kuma status page
(isitup.lotusguild.org/status/matrix), managed from Kuma's UI. The client
asks Kuma directly (not via our servers) so it still hears "the server is
down" when our servers can't tell it.

- config.json `statusPages`, keyed by homeserver: users of other servers
  never contact Kuma.
- utils/kumaStatus.ts (pure, unit-tested): parse Kuma 2.x's public JSON;
  a group is down when any monitor fails two checks in a row (down+down or
  pending+down); maintenance = windows under way; announcements = incidents.
  One strip at a time: server down (connection lost AND Kuma confirms) >
  server having problems > maintenance > calls down > announcement;
  sign-in problems on the login screen only.
- Wording about the user's own connection: "Connection lost … our status
  checks say the server is up, so it may be your internet connection" ONLY
  when Kuma checked the server after this client's connection dropped and
  it passed; a stale "up" (Kuma needs a minute or two to notice an outage)
  keeps the plain "Connection Lost!".
- useServerStatus: polls only while visible; 5 min, 60 s while something is
  wrong or the connection is lost, at once when it drops; backoff; any
  failure = no banner (Kuma being unreachable never looks like Matrix
  being down); GET only, no cookies.
- UI in the existing banner slot and style (ContainerColor/Line like the
  sync and clock banners); Details expands; dismiss for calls-down and
  announcements (an edited announcement comes back); calls-down note above
  Join; phone: one line + Details.

Needs the CSP connect-src to allow https://isitup.lotusguild.org (matrix
repo) before it can fetch in production; until then it fails quiet.

Tests: 15 unit tests (live page layout, two-check rule, any-monitor rule,
unknown/garbage, UTC beat times, stale-vs-fresh "up", priorities, login vs
client, maintenance, announcements + dismiss/edit); e2e (fixtures for Kuma):
calls-down strip + dismiss across reload, other homeservers make no
requests, Kuma 500 → nothing, lost connection + Kuma down → critical strip
instead of "Connection Lost", + fresh "up" → "may be your connection",
+ stale "up" → plain "Connection Lost". Unit 1315, Playwright 26 passed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/call/PrescreenControls.tsx

@@ -8,6 +8,7 @@ import { useCallEmbed, useCallJoined, useCallStart } from '../../hooks/useCallEm
 import { useCallPreferences } from '../../state/hooks/callPreferences';
 import { useSetting } from '../../state/hooks/settings';
 import { settingsAtom } from '../../state/settings';
+import { useCallsDown } from '../server-status/ServerStatusBanner';
 
 type MediaPermState = 'granted' | 'denied' | 'prompt' | 'unknown';
 
@@ -61,6 +62,7 @@ export function PrescreenControls({ canJoin }: PrescreenControlsProps) {
 
   const micPermission = useMediaPermissions();
   const micDenied = micPermission === 'denied';
+  const callsDown = useCallsDown();
 
   const disabled = inOtherCall || !canJoin || micDenied;
 
@@ -91,6 +93,11 @@ export function PrescreenControls({ canJoin }: PrescreenControlsProps) {
         <ChatButton />
       </Box>
       <Box grow="Yes" direction="Column" gap="200">
+        {callsDown && (
+          <Text size="T200" style={{ color: color.Warning.Main, textAlign: 'center' }}>
+            Voice calls are down right now, so joining may fail.
+          </Text>
+        )}
         {micDenied && (
           <Text size="T200" style={{ color: color.Critical.Main, textAlign: 'center' }}>
             Microphone access is blocked. Enable it in your browser settings to join.