feat(opencode): Phase 1 - Backend data structure expansion for OpenCode support

Add OpenCode as the 4th supported application with additive provider management:

- Add OpenCode variant to AppType enum with all related match statements
- Add enabled_opencode field to McpApps and SkillApps structures
- Add opencode field to McpRoot and PromptRoot
- Add database schema migration v3→v4 with enabled_opencode columns
- Add settings.rs support for opencode_config_dir and current_provider_opencode
- Create opencode_config.rs module for config file I/O operations
- Update all services (proxy, mcp, skill, provider, stream_check) for OpenCode
- Add OpenCode support to deeplink provider and MCP parsing
- Update commands/config.rs for OpenCode config status and paths

Key design decisions:
- OpenCode uses additive mode (no is_current needed, no proxy support)
- Config path: ~/.config/opencode/opencode.json
- MCP format: stdio→local, sse/http→remote conversion planned
- Stream check returns error (not yet implemented for OpenCode)

src-tauri/src/proxy/providers/mod.rs

@@ -132,6 +132,10 @@ impl ProviderType {
                 }
                 ProviderType::Gemini
             }
+            AppType::OpenCode => {
+                // OpenCode doesn't support proxy, but return a default type for completeness
+                ProviderType::Codex // Fallback to Codex-like type
+            }
         }
     }
 
@@ -176,6 +180,10 @@ pub fn get_adapter(app_type: &AppType) -> Box<dyn ProviderAdapter> {
         AppType::Claude => Box::new(ClaudeAdapter::new()),
         AppType::Codex => Box::new(CodexAdapter::new()),
         AppType::Gemini => Box::new(GeminiAdapter::new()),
+        AppType::OpenCode => {
+            // OpenCode doesn't support proxy, fallback to Codex adapter
+            Box::new(CodexAdapter::new())
+        }
     }
 }