dashboard: fire-now button on schedule rows (closes #467)

Operator wants to trigger a scheduled prompt immediately
instead of waiting for the next interval. Adds a `↯ fire now`
button on every active schedule row, next to `✕ cancel all`.

Semantics (per design discussion with damocles):

- recurring schedules → out-of-band pulse, `next_fire_at_unix`
  untouched; the regular cadence keeps firing on the original
  schedule. Operator gets an extra fan-out, not a phase shift.
- one-shots → consumed after the manual fire. Operator's
  intent reads as "the scheduled time was wrong, send NOW";
  leaving the original time would be surprising.

Confirm dialog spells out the recurring-vs-one-shot
difference up front so the operator knows what they're about
to do. Button is disabled when every target is already
cancelled (nothing to fire).

Talks to `POST /api/schedules/{id}/fire-now` (damocles is
wiring the backend in parallel). Mauve styling distinguishes
it from cancel (red) and submit (amber); fits the existing
btn pattern.

docs/web-ui.md updated.

frontend/packages/dashboard/src/dashboard.css

@@ -850,6 +850,7 @@ 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); }
+.btn-fire-now { color: var(--mauve, #cba6f7); border-color: var(--mauve, #cba6f7); }
 /* #474: inline edit button on each schedule row. Yellow reads as a
    parallel destructive-adjacent action (edit changes state, but
    isn't deletion). */