fix(examples): forward abort signal in tanstack-db-web-starter Electric proxy

[Jaime02]

Problem

proxyElectricRequest calls fetch(originUrl) without forwarding the incoming request's AbortSignal. When a client goes away mid-request — a page reload, a navigation, a closed tab — the proxy's upstream call to Electric has no cancellation path and keeps running to completion.

For live=true long-polls that is the full long-poll timeout. Measured against a local Electric (electricsql/electric:latest, default settings), proxying through a TanStack Start route:

unsignalled upstream long-poll held open for: 20004 ms  (status 200)
signalled upstream released after:                1 ms  (AbortError)

Each abandoned live request pins one upstream connection and one fetch slot for ~20s after the client that wanted it is already gone.

This compounds with the concurrency limit that skills/electric-proxy-auth/SKILL.md already documents:

Bun caps simultaneous fetch() calls at 256 per process by default. Excess requests queue silently — the proxy keeps accepting inbound connections but upstream calls to Electric stall, surfacing as latency spikes rather than errors.

A page holding ~10 shapes leaks ~10 slots per reload, each for ~20s. Roughly 26 abandoned page loads inside one 20-second window is enough to exhaust the pool — easily reached when a deploy causes many connected clients to reconnect at once. The symptom is the latency spike the skill describes; the missing signal is one of its causes.

Change

Forward the request's AbortSignal to the upstream fetch:

export async function proxyElectricRequest(
  originUrl: URL,
  signal?: AbortSignal
): Promise<Response> {
  const response = await fetch(originUrl, { signal })

and pass request.signal at the three call sites (todos, users, projects).

The parameter is optional, so this is source-compatible with any existing caller.

Same pattern elsewhere

I kept this PR to the one starter to stay reviewable, but the same fetch(originUrl) without a signal appears in:

• examples/tanstack-db-expo-starter/api/index.ts (~line 138)
• website/docs/sync/guides/auth.md (proxy examples around lines 152, 407, 414, 624, 644)
• packages/typescript-client/skills/electric-proxy-auth/SKILL.md (~line 51)