fix(translate): enforce 1500-char text limit + add request timeout

[missuo]

Summary

Two related stability fixes after we hit them in real-world use of the v1.2.x oneshot path.

1. Hung requests on slow upstream

Without an explicit HTTP timeout, a stuck DeepL connection would dangle indefinitely. Browser extensions calling `/translate` would sit on a spinner forever with no error to surface to the user.

Fix: `req.SetTimeout(20s)` on the oneshot client. On timeout return HTTP `504 Gateway Timeout` (distinguishable from generic 503 — "DeepL was slow" vs "connection refused / DNS failure / etc."). The check catches both `context.DeadlineExceeded` and `url.Error{Timeout()=true}` so it works whether the deadline came from `httpClient.Timeout` or a per-request `SetContext`.

The cookie-jar warmup GET to `www.deepl.com` gets its own shorter 5s timeout — warmup is best-effort, no point making the very first translate call wait 20s for a hung warmup.

2. No-feedback on oversized input

The oneshot endpoint hard-caps the total `text` array length at 1500 characters (verified upstream — matches the extension's own `G.notLoggedIn = 1500` constant in background.js). Past it, DeepL returns:

```json
{
"status": 400,
"errors": {"text": ["text exceeds maximum length"]},
"text_character_limit": 1500
}
```

We were happily forwarding any payload upstream, wasting a round trip, and the caller had to parse a generic 503 to figure out what went wrong.

Fix: Pre-validate `text` length in characters (`utf8.RuneCountInString`, not byte length — verified the upstream cap is rune-based: 1500 Chinese characters / 4500 bytes is accepted, 1501 is rejected). Return HTTP 413 Payload Too Large with a clear message naming the observed length and the limit:

```json
{
"code": 413,
"message": "text exceeds maximum length: 1501 characters (anonymous oneshot limit is 1500)"
}
```

Verification

Against the live oneshot endpoint:

• 1500 ASCII chars → 200
• 1501 ASCII chars → 413 (upstream not contacted — observed in dev server logs)
• 1500 Chinese chars (4500 UTF-8 bytes) → 200
• 1501 Chinese chars → 413
• Pathological "你"×1500 → 504 at exactly ~20s (was hanging without timeout)
• Realistic 245-char Chinese paragraph → 200 in ~13s
• Existing /translate /v1/translate /v2/translate routes unaffected for in-spec payloads

Status code summary

When	Code
Translation succeeded	200
target_lang / source_lang invalid	400
text empty	404
text > 1500 chars	413 (new)
DeepL took longer than 20s	504 (new)
DeepL rate-limited us	429
Other upstream failure	503