dashboard: inline edit form on schedule rows (closes #474)

Per damocles's PATCH /api/schedules/{id} backend (#475), each
non-cancelled schedule row gets a `✎ edit` button that toggles
an inline form pre-filled with current values. Editable:

- body (textarea)
- description (blank to clear)
- next-fire (datetime-local)
- interval (shared composer from #466 — all-zero flips to one-shot)

Targets stay immutable per the design call with damocles:
per-target last-result history is keyed on them; cancel + new
schedule is the documented retarget workaround. The form
surfaces the active target list as a read-only note explaining
this.

Submit semantics — the form computes a PATCH diff against the
original schedule and only includes keys for fields that
actually changed. Blank description → `null` (clear), all-zero
interval on a recurring schedule → `null` (flip to one-shot).
No-op submit (no fields changed) just closes the edit form.

Refactored the interval composer (#466) into a shared
`buildIntervalComposer({ label, namePrefix, initialSeconds })`
helper so the new-schedule and edit-schedule forms use the same
chip/d/h/m/s/preview widget. New-schedule form behavior
unchanged; edit-form input names are prefixed `edit_interval_`
to avoid FormData collisions when both happen to mount.

State survives state-poll re-renders via `editingSchedules`
(Set of ids being edited) + `scheduleEditCarry` (per-id
mid-edit values) — same pattern the new-schedule form uses
with `readScheduleFormCarry`.

frontend/packages/dashboard/src/dashboard.css

@@ -850,6 +850,10 @@ ul form.inline { display: inline-block; }
 .btn-start   { color: var(--green); border-color: var(--green); font-size: 0.75em; padding: 0.15em 0.5em; margin-left: 0.6em; }
 .btn-talk { color: var(--cyan); border-color: var(--cyan); }
 .btn-spawn { color: var(--amber); border-color: var(--amber); }
+/* #474: inline edit button on each schedule row. Yellow reads as a
+   parallel destructive-adjacent action (edit changes state, but
+   isn't deletion). */
+.btn-edit-schedule { color: var(--yellow, #f9e2af); border-color: var(--yellow, #f9e2af); }
 .spawnform { display: flex; gap: 0.6em; align-items: stretch; margin: 0.5em 0; }
 .spawnform input {
   font-family: inherit;
@@ -1604,7 +1608,8 @@ body.flow-shell .tabbar .tab.active.tab-link {
    Cards show: id + source + due-in + cancel-all in the header,
    the prompt body, then a targets table with per-row cancel. */
 
-.schedule-new-form {
+.schedule-new-form,
+.schedule-edit-form {
   display: flex;
   flex-direction: column;
   gap: 0.6em;
@@ -1614,6 +1619,20 @@ body.flow-shell .tabbar .tab.active.tab-link {
   padding: 0.8em 1em;
   margin-bottom: 0.5em;
 }
+/* #474 — inline edit form opens directly under the schedule row's
+   actions strip, indented slightly so it visually nests under the
+   row it edits. */
+.schedule-edit-form-wrapper {
+  margin-top: 0.5em;
+  padding-left: 0.5em;
+  border-left: 2px solid var(--yellow, #f9e2af);
+}
+.schedule-edit-targets-note {
+  color: var(--muted);
+  font-style: italic;
+  font-size: 0.85em;
+  margin: 0.2em 0 0 0;
+}
 .schedule-field {
   display: flex;
   flex-direction: column;