feat(translator): add Baidu-general-translate backend

[Gilgamesh-lzq]

The Service preset dropdown gains a "Baidu API" entry. Layout is unchanged — existing inputs are reused with per-kind labels, backed by a new providerKind discriminator on settings that defaults to "openai" so pre-existing data.json still loads.

Settings/UI:

• API key holds the Baidu secret; Base URL holds the appid; Model is displayed disabled with a note that it's ignored (no such wire field).
• cacheModel() pins the Baidu cache identity to a stable constant, so whatever sits in the ignored Model input can never drift the cache.
• matchPreset() matches by providerKind first (baseUrl is an appid for Baidu, not a URL); switching to Custom flips providerKind back to "openai".

Provider (pure, unit-tested):

• Ships a small MD5 (RFC 1321, UTF-8 aware) in-tree — Node's crypto is absent in the Obsidian renderer and SubtleCrypto omits MD5. Used ONLY for Baidu's non-cryptographic sign = md5(appid + q + salt + secret), computed over the raw q per the docs' 54001 troubleshooting.
• BaiduProvider aligns with the DeepSeek provider: same TranslationProvider surface, \n-join batching with per-segment
  line-count regroup, and per-segment fallback on SegmentCountMismatchError.
• Baidu error_code maps to the existing typed errors (52003/54001/… → Auth, 54003/54005/52001/52002 → RateLimit, else → Malformed).

Wiring:

• New createProvider(settings, http) factory; the settings-tab Test button and both translateButton.ts entry points route through it instead of constructing DeepSeekProvider directly.
• Baidu preset ships a standard-tier shape (concurrency 1, minIntervalMs 1100, small batches) so QPS=1 accounts pass untuned.