dashboard: friendlier interval input for scheduled prompts (closes #466)

Replaces the single raw-seconds field in the SCH3DUL3D PR0MPTS
creation form with a structured composer:

- Preset chip row (1m / 5m / 15m / 30m / 1h / 6h / 12h / 1d / 7d
  / one-shot) that fills the parts inputs in one click
- Four small d/h/m/s number inputs combined into total seconds
  on submit (all-zero = one-shot, preserving the backend null
  semantic)
- Live "↻ every …" preview using the existing fmtDuration helper
  so the operator sees what they're about to queue

Carry semantics extended to round-trip the split fields across
state-poll re-renders; legacy `interval` carry still honoured if
an older bundle is in the page.

Docs (web-ui.md) updated to describe the new composer.

frontend/packages/dashboard/src/dashboard.css

@@ -1665,6 +1665,50 @@ body.flow-shell .tabbar .tab.active.tab-link {
   justify-content: flex-end;
 }
 
+/* Interval composer (#466). Preset chip row + d/h/m/s inputs + live
+   preview, so the operator never has to multiply seconds by hand. */
+.schedule-interval-presets {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.3em;
+  margin-top: 0.1em;
+}
+.btn-interval-preset {
+  color: var(--cyan);
+  border-color: var(--cyan);
+  font-size: 0.75em;
+  padding: 0.15em 0.55em;
+  letter-spacing: 0.05em;
+}
+.btn-interval-oneshot {
+  color: var(--muted);
+  border-color: var(--muted);
+}
+.schedule-interval-parts {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 0.35em;
+  margin-top: 0.4em;
+}
+.schedule-interval-num {
+  width: 4em;
+  text-align: right;
+  font-variant-numeric: tabular-nums;
+}
+.schedule-interval-unit {
+  color: var(--muted);
+  font-size: 0.85em;
+  margin-right: 0.2em;
+}
+.schedule-interval-preview {
+  margin-top: 0.4em;
+  color: var(--cyan);
+  font-size: 0.9em;
+  font-variant-numeric: tabular-nums;
+}
+.schedule-interval-preview-oneshot { color: var(--muted); font-style: italic; }
+
 .schedules {
   display: flex;
   flex-direction: column;