fix: a stalled server no longer reads as "your clock is ahead"

Incident 2026-09-29: the homeserver's host ran out of memory and stalled for
~2 minutes. The /sync that finally went out carried events whose `age` was
computed ~30 s before it arrived, so every client showed "Your computer's
clock is 30 seconds ahead of the server" while the real problem was the
server (all host clocks were within 0.25 s the whole evening).

The skew estimate was the median of the last 5 samples, and a sample is
local skew + delivery delay, so one late /sync with a handful of events
tripped it.

- Estimate = the LOWEST sample of the last 5 minutes: delay only ever adds,
  so the fastest-delivered event is the truest.
- "Behind" (which a delay can't cause) is reported as soon as there are 3
  samples, like before. "Ahead" must hold across samples received at least
  a minute apart, so a single late burst never trips it.
- Samples are aged on the monotonic clock, and a change of the local clock
  (someone fixing it) resets the measurement, so the warning clears at once.
- Only events stamped by our own homeserver are sampled: a federated event's
  origin_server_ts is the other server's clock.
- Wording: "This device's clock is … Voice calls and encrypted messages can
  fail until it's corrected." / call bar "Device clock … : calls may fail"
  (was "will fail").

Unit tests: the incident (late burst after normal traffic, and a fresh
client whose first samples are all late), mixed slow/fast deliveries,
ahead only after a minute, behind at once, hysteresis, clock fixed.

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

src/app/pages/client/ClientNonUIFeatures.tsx

@@ -1071,6 +1071,10 @@ function ClockSkewFeature() {
       data,
     ) => {
       if (!data.liveEvent) return;
+      // Only events our homeserver stamped: a federated event's
+      // origin_server_ts is the other server's clock.
+      const senderServer = mEvent.getSender()?.split(':').slice(1).join(':');
+      if (senderServer !== mx.getDomain()) return;
       monitor.sample(mEvent.getTs(), mEvent.getAge(), mEvent.localTimestamp);
     };
     mx.on(RoomEvent.Timeline, onTimeline);

src/app/pages/client/ClockSkewBanner.tsx

@@ -19,7 +19,7 @@ const readDismissedUntil = (): number => {
 };
 
 /**
- * [Gitea #158] "Your computer's clock is 14 minutes ahead of the server."
+ * [Gitea #158] "This device's clock is 14 minutes ahead of the server."
  * Same slot and style as the sync banners. Shown while the skew monitor is
  * over its threshold; the direction matters, so it is said. Dismissable for
  * 24 h; never auto-corrects anything.
@@ -53,8 +53,8 @@ export function ClockSkewBanner() {
       >
         <Box alignItems="Center" gap="300" wrap="Wrap" justifyContent="Center">
           <Text size="L400" align="Center">
-            Your computer&apos;s clock is <b>{describeSkewVsServer(skewMs)}</b>. Encrypted messages
-            and voice calls will fail until it is fixed.
+            This device&apos;s clock is <b>{describeSkewVsServer(skewMs)}</b>. Voice calls and
+            encrypted messages can fail until it&apos;s corrected.
           </Text>
           <Button
             size="300"