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/call.js

@@ -15,6 +15,7 @@
 // registers once via `initCall`.
 
 import { $, el, form, Panel, appendLinkified } from './common.js';
+import { themedToast } from './modal.js';
 import { fmtAgo } from './util.js';
 import { questionsState, QUESTION_HISTORY_LIMIT } from './state.js';
 
@@ -575,7 +576,7 @@ function buildQuestionLi(q) {
     const existing = f.querySelector('input[name="answer"]');
     if (existing) existing.remove();
     f.append(el('input', { type: 'hidden', name: 'answer', value: merged }));
-    if (!merged) { ev.preventDefault(); alert('pick an option or type an answer'); }
+    if (!merged) { ev.preventDefault(); themedToast('pick an option or type an answer', { type: 'error' }); }
   }, true);
   if (hasOptions) f.append(optionGroup);
   const buttons = el('div', { class: 'q-buttons' });