feat(heph-tui): show sync age in seconds under a minute

The background sync loop runs every 30s, so the last-sync age never crossed
the 60s 'just now' threshold — the chip always read 'just now', which also
masked the first missed sync (age 30-60s looked identical to a fresh one).
Show seconds under a minute ('⟳ 26s') so the chip is a visible heartbeat and a
stalled sync surfaces ~30s sooner.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/heph-tui/src/fmt.rs

@@ -30,13 +30,15 @@ pub fn now_ms() -> i64 {
     Local::now().timestamp_millis()
 }
 
-/// A compact "how long ago" for the sync indicator: `just now` under a minute,
-/// then `Nm` / `Nh` / `Nd`. Clamped at zero so a little clock skew never shows a
-/// negative age.
+/// A compact "how long ago" for the sync indicator: `Ns` under a minute, then
+/// `Nm` / `Nh` / `Nd`. Second-granularity under a minute makes the chip a visible
+/// heartbeat (the sync loop runs every 30s) and surfaces a missed beat as the age
+/// climbing, rather than hiding under a flat "just now". Clamped at zero so a
+/// little clock skew never shows a negative age.
 pub fn fmt_age(now_ms: i64, then_ms: i64) -> String {
     let secs = (now_ms - then_ms).max(0) / 1000;
     if secs < 60 {
-        "just now".into()
+        format!("{secs}s")
     } else if secs < 3_600 {
         format!("{}m", secs / 60)
     } else if secs < 86_400 {
@@ -126,13 +128,14 @@ mod tests {
     #[test]
     fn age_is_compact_and_clamped() {
         let now = 1_000_000_000_000;
-        assert_eq!(fmt_age(now, now), "just now");
-        assert_eq!(fmt_age(now, now - 30_000), "just now");
+        assert_eq!(fmt_age(now, now), "0s");
+        assert_eq!(fmt_age(now, now - 30_000), "30s");
+        assert_eq!(fmt_age(now, now - 59_000), "59s");
         assert_eq!(fmt_age(now, now - 5 * 60_000), "5m");
         assert_eq!(fmt_age(now, now - 3 * 3_600_000), "3h");
         assert_eq!(fmt_age(now, now - 2 * 86_400_000), "2d");
         // Clock skew (then in the future) never shows a negative age.
-        assert_eq!(fmt_age(now, now + 10_000), "just now");
+        assert_eq!(fmt_age(now, now + 10_000), "0s");
     }
 
     #[test]

crates/heph-tui/src/ui.rs

@@ -661,10 +661,7 @@ mod tests {
             last_success_ms: Some(NOW),
             ..Default::default()
         };
-        assert_eq!(
-            render(&spoke(h.clone(), 1), NOW),
-            "⟳ just now  ⚠ 1 conflict"
-        );
-        assert_eq!(render(&spoke(h, 3), NOW), "⟳ just now  ⚠ 3 conflicts");
+        assert_eq!(render(&spoke(h.clone(), 1), NOW), "⟳ 0s  ⚠ 1 conflict");
+        assert_eq!(render(&spoke(h, 3), NOW), "⟳ 0s  ⚠ 3 conflicts");
     }
 }

docs/changelog.d/+sync-age-seconds.feature.md

@@ -0,0 +1 @@
+heph-tui's sync indicator now shows the last-sync age in seconds under a minute (`⟳ 26s`) instead of a flat `just now`, so the chip reads as a live heartbeat and a missed sync (the loop runs every 30s) shows up as the age climbing.