feat(run-page): tick the run-duration timer between SSE pushes

Adds a 1s client-side ticker that rewrites .run-duration text from a
data-started-at attribute, so the header timer on /runs/{id}
increments every second while the run is active. When an SSE swap
lands a fresh header the new server-rendered value seamlessly takes
over; when the run goes terminal the template drops the attribute
and the ticker silently skips the node, leaving the final elapsed in
place.

Other templ_*.go churn is cosmetic — regenerator versions differ
between CI and local and only the filename field in templ.Error
callsites changed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/web/static/app.js

@@ -84,7 +84,42 @@
         });
     });
 
-    // --- 3. permalink scroll + highlight on load ------------------------
+    // --- 3. live duration tick ------------------------------------------
+    //
+    // .run-duration spans carry data-started-at (RFC3339) while the run is
+    // non-terminal. Every second we rewrite their text with the current
+    // elapsed so the header timer ticks between SSE pushes. When an SSE
+    // swap drops the attribute (run finished), the tick silently skips it
+    // and the server-rendered final value stays put.
+
+    function formatDuration(ms) {
+        if (ms < 0) { ms = 0; }
+        if (ms < 1000) { return Math.floor(ms) + 'ms'; }
+        if (ms < 10000) { return (ms / 1000).toFixed(1) + 's'; }
+        if (ms < 60000) { return Math.floor(ms / 1000) + 's'; }
+        if (ms < 3600000) {
+            var m = Math.floor(ms / 60000);
+            var s = Math.floor((ms % 60000) / 1000);
+            return m + 'm ' + s + 's';
+        }
+        var h = Math.floor(ms / 3600000);
+        var mm = Math.floor((ms % 3600000) / 60000);
+        return h + 'h ' + mm + 'm';
+    }
+
+    function tickDurations() {
+        var now = Date.now();
+        var nodes = document.querySelectorAll('.run-duration[data-started-at]');
+        nodes.forEach(function (el) {
+            var startMs = Date.parse(el.getAttribute('data-started-at'));
+            if (isNaN(startMs)) { return; }
+            el.textContent = formatDuration(now - startMs);
+        });
+    }
+
+    setInterval(tickDurations, 1000);
+
+    // --- 4. permalink scroll + highlight on load ------------------------
 
     function scrollToHash() {
         var hash = (location.hash || '').replace(/^#/, '');