feat: add OpenCode Zen and Go as provider options (#26)

- Add OpenCode Zen (https://opencode.ai/zen/v1) and Go (https://opencode.ai/zen/go/v1)
- Add provider detection via _host_match() in llm_core.py
- Add curated model list entries in model_routes.py
- Add webhook provider URLs
- Add provider icon (providers.js) and dropdown options (index.html)
- Add auto-detection patterns and setup URLs (slashCommands.js)
- Whitelist opencode.ai in URL validation (admin.js)
- Rebased on main to fix merge conflicts with _HOST_TO_CURATED refactor

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

routes/model_routes.py

@@ -285,6 +285,8 @@ _HOST_TO_CURATED = (
     ("x.ai", "xai"),
     ("openrouter.ai", "openrouter"),
     ("ollama.com", "ollama"),
+    ("opencode.ai/zen/go", "opencode-go"),
+    ("opencode.ai/zen", "opencode-zen"),
 )