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/modal.css

@@ -1,111 +0,0 @@
-/* modal.css — themed dialog component styles.
-   Pairs with modal.js (themedConfirm / themedPrompt / themedToast). The
-   dialogs are raised from common.js's data-async / data-confirm handler,
-   which every page loads, so common.css @imports this to keep the styles
-   present wherever a dialog can fire — not just the main dashboard.
-   (Previously these rules lived in dashboard.css, so dialogs rendered
-   unstyled on standalone pages such as /core.) */
-
-.tc-backdrop {
-  position: fixed;
-  inset: 0;
-  z-index: 1000;
-  display: flex;
-  align-items: center;
-  justify-content: center;
-  padding: 1em;
-  background: color-mix(in srgb, var(--crust) 60%, transparent);
-  -webkit-backdrop-filter: blur(2px);
-  backdrop-filter: blur(2px);
-}
-.tc-box {
-  background: var(--bg-elev);
-  border: 1px solid var(--purple-dim);
-  box-shadow: 0 8px 40px -8px var(--crust);
-  padding: 1.2em 1.4em;
-  max-width: min(32em, 92vw);
-  display: flex;
-  flex-direction: column;
-  gap: 0.9em;
-}
-.tc-title {
-  font-weight: bold;
-  text-transform: uppercase;
-  letter-spacing: 0.08em;
-  color: var(--subtext0);
-}
-.tc-message { color: var(--fg); line-height: 1.45; }
-.tc-checks { display: flex; flex-direction: column; gap: 0.4em; }
-.tc-checkrow {
-  display: flex;
-  align-items: flex-start;
-  gap: 0.5em;
-  cursor: pointer;
-  color: var(--subtext0);
-  font-size: 0.92em;
-  line-height: 1.35;
-}
-.tc-checkrow .tc-check { margin-top: 0.2em; flex: 0 0 auto; }
-.tc-actions {
-  display: flex;
-  justify-content: flex-end;
-  gap: 0.6em;
-  margin-top: 0.2em;
-}
-.tc-cancel { color: var(--subtext0); }
-.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); }
-/* themedPrompt's field is always a resizable textarea (Enter submits,
-   Shift+Enter inserts a newline) — sensible min-height, wrapped output. */
-.tc-textarea {
-  resize: vertical;
-  min-height: 4.5em;
-  line-height: 1.4;
-  white-space: pre-wrap;
-}
-
-/* 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); }