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/dashboard/src/common.css

@@ -4,7 +4,7 @@
 @import "@hive/shared/terminal.css";
 @import "@hive/shared/tabs.css";
 @import "@hive/shared/chrome.css";
-@import "./modal.css";
+@import "@hive/shared/modal.css";
 
 /* ─── global typography ─────────────────────────────────────────────
    Element-level rules shared across all three pages (index, flow,