feat(providers): support OpenAI-compatible / Azure / Google Gemini endpoints (closes #16)

The hosted-API BYOK fallback was hard-wired to api.anthropic.com. Issue
#16 asks for Azure (Microsoft OpenAI hosting Anthropic-style models) but
the same plumbing unlocks every common BYOK target — OpenRouter,
LiteLLM, DeepSeek, Groq, Together, Mistral, plus Google Gemini direct.

Provider model:
- New `provider: 'anthropic' | 'openai' | 'azure' | 'google'` discriminator
  on AppConfig (defaults to 'anthropic' so existing localStorage
  configs migrate seamlessly).
- src/providers/model.ts routes to one of four streaming clients:
  anthropic.ts (existing SDK path), openai.ts (new SSE pump shared with
  azure.ts), azure.ts (deployment URL + api-key header), google.ts
  (Generative Language streamGenerateContent).
- src/providers/presets.ts ships per-provider defaults (baseUrl, model
  suggestions, api-key placeholder, Azure api-version flag) so the UI
  can stay declarative.

UI:
- SettingsDialog now shows a provider picker on the Hosted-API tab and
  surfaces an Azure-only api-version field. Provider switches preserve
  any non-empty user values.
- EntryView / AvatarMenu env meta line shows the active provider label.
- en + zh-CN locales updated; README + README.zh-CN document every
  provider, with explicit guidance to reach AWS Bedrock / GCP Vertex
  Anthropic models via a server-side LiteLLM proxy (signing belongs on
  the server, not the browser).

Why an OpenAI-compatible adapter rather than a native Bedrock/Vertex
client: AWS SigV4 and GCP service-account JWTs aren't safe to do from a
browser holding long-lived BYOK credentials. LiteLLM (or any
Anthropic/OpenAI-compatible proxy) sidesteps that and is the same path
lobe-chat uses for Bedrock/Vertex.

src/types.ts

@@ -1,10 +1,25 @@
 export type ExecMode = 'daemon' | 'api';
 
+// Which BYOK model endpoint to talk to in `mode === 'api'`. Each provider
+// has its own request shape — see src/providers/{anthropic,openai,azure,
+// google}.ts for the wire details. AWS Bedrock and Google Vertex are
+// reached via the `anthropic` provider pointed at an Anthropic-compatible
+// proxy (e.g. LiteLLM), which keeps signing on the server where the
+// long-lived AWS / GCP credentials belong.
+export type ModelProvider = 'anthropic' | 'openai' | 'azure' | 'google';
+
 export interface AppConfig {
   mode: ExecMode;
+  // Active provider when `mode === 'api'`. Older configs that predate the
+  // multi-provider rework default to 'anthropic' on load.
+  provider: ModelProvider;
   apiKey: string;
   baseUrl: string;
   model: string;
+  // Azure OpenAI only — the api-version query string the Azure REST
+  // surface requires (e.g. '2024-08-01-preview'). Ignored by every other
+  // provider so the same config can round-trip through localStorage.
+  apiVersion?: string;
   agentId: string | null;
   skillId: string | null;
   designSystemId: string | null;