security: clear all High next alerts by upgrading react-email 5 → 6 (#21377)

## What

Clears **all 9 High `next` Dependabot alerts** (incl.
GHSA-26hh-7cqf-hhc6) in twenty-emails — via a parent bump, no
resolutions.

All 9 traced to a stale **`next@16.0.10`** pulled by
`@react-email/preview-server` 5.x. The latest preview-server 5.x still
ships a vulnerable next (16.1.7 < 16.2.6), so bumping it alone wouldn't
help. **react-email 6.x** is a rewrite that no longer depends on next or
on a separate preview-server.

- bump `react-email` `5.1.0` → `6.5.0`
- remove the obsolete `@react-email/preview-server` devDependency
- add `@react-email/ui` `6.5.0` devDependency

### Why `@react-email/ui` (the CI fix)

react-email 6's `email dev` preview server loads its UI from
`@react-email/ui`, and **prompts to install it interactively** if
missing — which hangs the `emails-test` CI job (no TTY), so the server
never starts and the `/preview/test.email` smoke check fails. Pinning
`@react-email/ui` makes `email dev` start non-interactively.

### Net effect on `next`

The vulnerable `16.0.10` is gone. `@react-email/ui@6.5.0` pulls
**`next@16.2.6`** — the **patched** version (≥ every current next
advisory fix), so all 9 alerts clear and **no vulnerable next remains**.

## Notes
- `react-email` and `@react-email/ui` pinned to exact `6.5.0` (matching
the prior react-email pin) because the `6.6.0` line was published today
and is still registry-quarantined.
- react-email is a dev-only preview tool; CI builds emails via `vite` +
typecheck.

## Verification
- No `next < 16.2.6` in `yarn.lock`
- `nx build` + `nx typecheck` twenty-emails
- `email dev -d src/emails -p 4001` starts non-interactively and serves
`/preview/test.email` → HTTP 200 (reproduces the emails-test check, now
passing)
- `yarn install --immutable` clean

packages/twenty-emails/package.json

@@ -22,13 +22,13 @@
     "@lingui/cli": "^5.1.2",
     "@lingui/swc-plugin": "^5.11.0",
     "@lingui/vite-plugin": "^5.1.2",
-    "@react-email/preview-server": "5.1.0",
+    "@react-email/ui": "6.5.0",
     "@tiptap/core": "^3.4.2",
     "@types/react": "^19",
     "@types/react-dom": "^19",
     "@typescript/native-preview": "^7.0.0-dev.20260116.1",
     "@vitejs/plugin-react-swc": "4.2.3",
-    "react-email": "5.1.0",
+    "react-email": "6.5.0",
     "tsc-alias": "^1.8.16",
     "vite-plugin-dts": "^4.5.4"
   },