feat(providers): cc-switch 式供应商自动故障转移（同厂商同模型） - #385

…viders

Adds automatic provider failover for chat requests, configured per vendor
tab in provider settings and modeled after cc-switch's proxy-side design:

- Failover switches providers, never models: the failed request is re-sent
  to the next provider in the queue with the same model id the conversation
  was using; queued providers without that model active are skipped.
- Per-target circuit breaker (consecutive-failure threshold, cooldown with
  half-open probe) keyed by provider::model, plus an error classifier that
  only fails over on provider-fault-class errors (5xx/network/auth/quota),
  never on client-request-class errors (context overflow etc.).
- Stream wrapper buffers events until first committed content so consumers
  never see output from a discarded attempt; sticky winner keeps follow-up
  rounds on the provider that answered, and onSwitched pins the conversation
  selection to it.
- Settings: modelFailover is now persisted (it previously round-tripped
  through neither the SQLite nor the localStorage channel, so the toggle
  and queue silently reset on every settings reopen); queue entries are
  provider ids with legacy {customProviderId, model} entries migrated by
  collapsing to their provider id.
- UI: failover card under each vendor tab manages an ordered provider
  queue with breaker knobs; i18n zh/en updated, {vendor} placeholder now
  replaced at every occurrence.

Covered by unit tests for the breaker/stream wrapper, plan builder, and
settings normalization/migration; verified end-to-end in the running app.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

…s drawer

The failover card no longer renders under each vendor's provider list; it
now lives in the drawer behind the gear button in the providers header,
scoped to the active vendor tab. The drawer is renamed from "Custom
Settings" to "Advanced Settings" (自定义设置 → 高级设置) in zh/en on both
the GUI and WebUI ends.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

The 3-column grid was cramped inside the 440px advanced-settings drawer;
one field per row reads better there.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>