refactor(dev): drop the Vite-under-Bun proxy workarounds Bun 1.4.1 made obsolete

[DavidBabinec]

What changed

Stacked on #502 (Bun 1.4.2). Deletes the three workarounds this repo carried for running the Vite dev server inside Bun:

• vite.config.ts forwards WebSocket upgrades on /admin/api (ws: true). The VITE_CMS_DEV_PORT define and the large-body proxy plugin are gone.
• The collab socket is same-origin in dev exactly as in production: window.location.host + SITE_SOCKET_PATH. socketUrl.ts, its test, and the import.meta.env.DEV special case are deleted.
• scripts/lib/largeBodyDevProxy.ts and its test are deleted.
• devWorkflow.test.ts gates the other way now: ws forwarding must stay on, and no CMS port may be dialled directly.
• docs/features/site-shell.md and docs/e2e/README.md updated.

Net: 386 lines removed, 94 added, no runtime behaviour change in production (which was already same-origin).

Why

Every one of those workarounds sat on the same seam: Bun's node:http client never emitted 'upgrade', its socket lacked destroySoon(), and its proxy could stop draining large bodies under backpressure. Bun 1.4.1 fixed the socket lifecycle (ws handleUpgrade after an await, paused sockets never emitting end, WebSocket backpressure stalls). On 1.4.2 the workarounds are dead weight, and the dev server finally behaves like production: one origin, one proxy, no port special cases.

Proof

Check	Result
bun run build / bun run lint / bun test under 1.4.2	clean, clean, 6827 pass
Collab socket upgrade through the Vite proxy (browser on the Vite port)	new WebSocket to ws://localhost:5291/admin/api/cms/site-socket opens in 6 ms; the editor reports Draft synced
Media upload larger than 1 MiB through the proxy	1.48 MiB PNG, HTTP 201 in 0.16 s, all 1,556,661 bytes received
Vite process alive after the socket closes	still serving after the editor and a probe socket closed; no destroySoon or uncaught error in the dev log
vite-dev-smoke on ubuntu-latest (Vite comes up; upgrade returns a status line)	success: Vite ready after 2 s under Bun 1.4.2, and the upgrade through the proxy returned HTTP/1.1 401 Unauthorized (the CMS refusing an anonymous socket) instead of hanging. The first E2E CI run had waited 120 s for this under 1.3.11.

The smoke workflow (.github/workflows/vite-dev-smoke.yml) is a throwaway for this branch and is removed before merge.