feat: view and edit a transaction's note from the list

Transactions can carry a `notes` field (set via the API — e.g. the
currency-conversion metadata on this user's imported historical data), but
there was no way to see it in the UI at all.

TransactionsTable now shows a note icon next to edit/delete, only when a
transaction actually has one, opening a dialog with the text. The edit
form's `notes` field was already wired end to end (state, payload) but had
no input in the JSX — added it, since a view-only note with no way to edit
or clear it would be a dead end.

Also fixed e2e/run.sh's readiness check, found while running the full
suite for this change: it polled `/` (nginx, answers almost immediately)
instead of `/api/health`, so the first test could start before the backend
finished migrating/seeding and fail trying to parse nginx's HTML error
page as JSON.

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "aurum-frontend",
   "private": true,
-  "version": "0.19.2",
+  "version": "0.20.0",
   "type": "module",
   "scripts": {
     "dev": "vite",