feat(opencode): Phase 9 - Frontend UI components for OpenCode

- Create OpenCodeFormFields.tsx with:
  - NPM package selector (from AI SDK ecosystem)
  - API Key input using shared ApiKeySection component
  - Base URL input (shown for openai-compatible)
  - Dynamic models editor (add/remove models)

- Update ProviderForm.tsx:
  - Import OpenCode presets and form fields
  - Add OPENCODE_DEFAULT_CONFIG constant
  - Add OpenCode to PresetEntry type union
  - Add OpenCode preset entries in useMemo
  - Add OpenCode state hooks (npm, apiKey, baseUrl, models)
  - Add OpenCode change handlers syncing to form
  - Add OpenCodeFormFields rendering section
  - Add OpenCode config editor using CommonConfigEditor

- Update ProviderActions.tsx for OpenCode additive mode:
  - Add appId and isInConfig props
  - Implement "Add to Config" / "Remove from Config" buttons
  - Disable failover mode for OpenCode
  - Update delete button logic for additive mode

- Update ProviderCard.tsx:
  - Pass appId and isInConfig to ProviderActions

- Update AddProviderDialog.tsx:
  - Add OpenCode base URL extraction from options.baseURL

src/components/providers/AddProviderDialog.tsx

@@ -17,6 +17,7 @@ import { UniversalProviderPanel } from "@/components/universal";
 import { providerPresets } from "@/config/claudeProviderPresets";
 import { codexProviderPresets } from "@/config/codexProviderPresets";
 import { geminiProviderPresets } from "@/config/geminiProviderPresets";
+// Note: opencodeProviderPresets is loaded via ProviderForm, not needed here
 import type { UniversalProviderPreset } from "@/config/universalProviderPresets";
 
 interface AddProviderDialogProps {
@@ -153,6 +154,7 @@ export function AddProviderDialog({
               }
             }
           }
+          // Note: OpenCode doesn't use endpointCandidates - it handles endpoints internally
         }
 
         if (appId === "claude") {
@@ -175,6 +177,12 @@ export function AddProviderDialog({
           if (env?.GOOGLE_GEMINI_BASE_URL) {
             addUrl(env.GOOGLE_GEMINI_BASE_URL);
           }
+        } else if (appId === "opencode") {
+          // OpenCode uses options.baseURL
+          const options = parsedConfig.options as Record<string, any> | undefined;
+          if (options?.baseURL) {
+            addUrl(options.baseURL);
+          }
         }
 
         const urls = Array.from(urlSet);