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/features/call-status/CallStatus.tsx

@@ -72,9 +72,9 @@ export function CallStatus({ callEmbed }: CallStatusProps) {
                     size="T200"
                     truncate
                     style={{ color: color.Warning.Main }}
-                    title="Fix your computer's clock — calls and encryption depend on it"
+                    title="This device's clock is off. Calls and encryption depend on it: turn on automatic time in your system settings."
                   >
-                    Clock {describeSkewVsServer(clockSkew.skewMs)} — calls will fail
+                    Device clock {describeSkewVsServer(clockSkew.skewMs)}: calls may fail
                   </Text>
                 </>
               )}