frontend: extract themed dialogs + async-form handler to shared, wire agent UI

The dashboard has a themed modal/dialog system (modal.js: themedToast/
themedConfirm/themedPrompt) and a data-async form submit interceptor
(bindAsyncForms) that every dashboard action routes through. The
per-agent UI never adopted either — it had its own more primitive
data-async handler using native window.confirm()/alert() (8 call
sites) and a duplicated el() DOM helper.

- Moved el() out of dashboard/common.js into shared/src/dom.js.
- Moved modal.js + modal.css from dashboard/src/ to shared/src/,
  updating its internal el import.
- Moved bindAsyncForms from dashboard/common.js into shared/forms.js,
  alongside the asyncBtn primitive it's built on.
- Updated every dashboard file's imports to the new shared locations
  (no re-export shims).
- agent.css now @imports shared/modal.css so the dialogs render
  themed there too.
- agent/app.js: dropped its local el()/data-async duplicate, wired
  bindAsyncForms(), and replaced all 8 window.confirm() sites with
  themedConfirm (async, wrapped in a fire-and-forget IIFE where the
  call site needs a synchronous boolean return, e.g. the slash-command
  dispatcher).

Closes hyperhive#2791. Verified with a full frontend build
(npm run build) — both dashboard and agent bundles compile clean and
agent.css picks up the .tc-* dialog styles it previously lacked.

frontend/packages/shared/src/dom.js

@@ -0,0 +1,23 @@
+// Tiny DOM-builder helper shared by the dashboard and the per-agent UI —
+// both packages built their own copy independently; this is the merged
+// canonical version (dashboard's, which had the extra `data-` branch;
+// functionally identical to the `class`/`html` handling either package
+// used).
+//
+// `el(tag, attrs, ...children)` creates an element, applying `attrs` as
+// either the `class`/`html` special cases or plain attributes, and
+// appending `children` (strings become text nodes, `null`/`undefined`
+// entries are skipped so callers can inline conditional children).
+export const el = (tag, attrs = {}, ...children) => {
+  const e = document.createElement(tag);
+  for (const [k, v] of Object.entries(attrs)) {
+    if (k === 'class') e.className = v;
+    else if (k === 'html') e.innerHTML = v;
+    else e.setAttribute(k, v);
+  }
+  for (const c of children) {
+    if (c == null) continue;
+    e.append(c.nodeType ? c : document.createTextNode(c));
+  }
+  return e;
+};