fix(studio): name the cause when a render request fails

[miguel-heygen]

What

Studio's render request now reports why it failed instead of collapsing every cause into
one sentence.

Why

useRenderQueue.ts wrapped the render POST in try { ... } catch { ... } — no binding, so
the exception was discarded:

} catch {
  error: "Could not reach render server. Use `hyperframes render` from the CLI instead.",

A dead server, a DNS failure, an aborted request and a server that died mid-render all
produce that identical string.

It is not only a UI message. It is the value that travels into the feedback report, so
three separate field reports arrived carrying it verbatim — one of them describing a
render that fails every single time. A guaranteed reproduction we cannot read is worse
than an intermittent one we can.

How

Bind the error, append it after the guidance:

} catch (err) {
  const cause = err instanceof Error ? err.message : String(err);
  ...
  error: `Could not reach render server: ${cause}. Use \`hyperframes render\` from the CLI instead.`,

The CLI suggestion stays — it is still the right next step for the user — it just no longer
stands alone.

Test plan

New useRenderQueueTransportError.test.tsx stubs fetch to reject with
TypeError("Failed to fetch") and asserts both halves of the contract: the cause appears,
and the guidance survives.

Verified it fails on the unfixed code rather than assuming:

× names the cause instead of collapsing every failure into one sentence
  → expected 'Could not reach render server. Use `h…' to contain 'Failed to fetch'

With the fix: 1 passed. Full packages/studio suite green at 4379 passed / 18 todo.

Not covered

• This does not fix any underlying cause of the reported failures; it makes the next report
  diagnosable. The "happens every time" case remains open until a report arrives carrying
  the real error.
• Only the transport catch is changed. The !res.ok branch below it already surfaces
  server-side detail and is untouched.