dashboard: extract themed dialog into a reusable modal.js component

Move the themed dialog out of common.js into its own modal.js module: a
general openDialog(title/message/content/buttons) primitive with themedConfirm
as a thin cancel/confirm wrapper on top. tabs.js imports it from there. No
behaviour change to the stop-confirm flow; the dialog is now a standalone
reusable component other surfaces can open.

frontend/packages/dashboard/src/tabs.js

@@ -14,10 +14,11 @@ import { marked } from 'marked';
 import {
   $, el, esc, form,
   fmtAgeSecs,
-  Panel, NOTIF, themedConfirm,
+  Panel, NOTIF,
   makePathLink, appendText, appendLinkified,
   openStream, renderServerWarnings, bindAsyncForms,
 } from './common.js';
+import { themedConfirm } from './modal.js';
 import { createTabStrip } from '@hive/shared/tabs.js';
 import {
   containersState, syncContainersFromSnapshot,