dashboard: theme all remaining dialogs (replace native confirm/alert/prompt)

Follow-up to the themed-modal component: route every remaining native
browser dialog through modal.js so nothing falls back to the OS chrome.

- modal.js: add themedPrompt (input dialog) + themedToast (non-blocking
  transient notification, info/error/ok) alongside openDialog/themedConfirm.
- Migrate call sites: bindAsyncForms confirm/prompt/alerts (common.js),
  the answer-validation alert (call.js), the M0V3 reparent confirm + action
  toasts (tabs.js), and the schedule form/cancel/fire confirms + validation
  and error alerts (schedules.js).
- UX: blocking modal for confirms/prompts; non-blocking toast for transient
  errors + validation. Destructive confirms keep the danger styling.
- CSS for the toast stack + prompt input.

common.js <-> modal.js is a safe deferred import cycle (usage is call-time
only); esbuild bundles it clean.

frontend/packages/dashboard/src/dashboard.css

@@ -467,6 +467,52 @@ body.dashboard-shell {
 .tc-confirm { color: var(--green); }
 .tc-confirm.tc-danger { color: var(--red); }
 
+/* themedPrompt input field */
+.tc-promptfield { display: flex; flex-direction: column; gap: 0.4em; }
+.tc-promptlabel { color: var(--subtext0); font-size: 0.9em; }
+.tc-input {
+  font-family: inherit;
+  font-size: 1em;
+  width: 100%;
+  box-sizing: border-box;
+  background: var(--crust);
+  color: var(--fg);
+  border: 1px solid var(--purple-dim);
+  padding: 0.4em 0.6em;
+}
+.tc-input:focus { outline: 1px solid var(--green); }
+
+/* themedToast — non-blocking transient notifications (errors/info/ok) */
+.tc-toasts {
+  position: fixed;
+  top: 1em;
+  right: 1em;
+  z-index: 1100;
+  display: flex;
+  flex-direction: column;
+  gap: 0.5em;
+  max-width: min(28em, 92vw);
+  pointer-events: none;
+}
+.tc-toast {
+  pointer-events: auto;
+  cursor: pointer;
+  background: var(--bg-elev);
+  border: 1px solid var(--purple-dim);
+  border-left-width: 3px;
+  box-shadow: 0 4px 20px -6px var(--crust);
+  padding: 0.6em 0.9em;
+  font-size: 0.9em;
+  color: var(--fg);
+  white-space: pre-wrap;
+  opacity: 1;
+  transition: opacity 0.2s ease, transform 0.2s ease;
+}
+.tc-toast-out { opacity: 0; transform: translateX(0.5em); }
+.tc-toast-error { border-left-color: var(--red); }
+.tc-toast-info { border-left-color: var(--purple-dim); }
+.tc-toast-ok { border-left-color: var(--green); }
+
 .agent-status {
   font-size: 0.82em;
   color: var(--subtext0);