feat: add OpenCode provider support (#387)

Adds OpenCode (https://github.com/opencode-ai/opencode) as a supported
provider. OpenCode is an open-source terminal AI coding assistant with
an OpenAI-compatible API running locally.

Changes:
- open-sse/config/providers.js: add opencode baseUrl (localhost:4096)
  with openai format (fully compatible, no custom headers needed)
- open-sse/services/model.js: add 'oc' alias → opencode
- src/shared/constants/providers.js: add opencode to subscription
  providers with alias 'oc', icon 'terminal', color #E87040

Usage after setup: use model prefix 'oc/<model>' to route through
a running OpenCode instance (e.g. oc/claude-sonnet-4-5).

Closes #378

open-sse/config/providers.js

@@ -208,6 +208,11 @@ export const PROVIDERS = {
     format: "openai",
     headers: {}
   },
+  opencode: {
+    baseUrl: "http://localhost:4096/v1/chat/completions",
+    format: "openai",
+    headers: {}
+  },
   cline: {
     baseUrl: "https://api.cline.bot/api/v1/chat/completions",
     format: "openai",

open-sse/services/model.js

@@ -12,6 +12,7 @@ const ALIAS_TO_PROVIDER_ID = {
   kc: "kilocode",
   kmc: "kimi-coding",
   cl: "cline",
+  oc: "opencode",
   // API Key providers
   openai: "openai",
   anthropic: "anthropic",