feat(integrations): add merge-method selection and provider-specific payloads

Introduce dedicated merge handling for integration review merges:

- Add src-tauri/src/integrations/merge.rs: implements merge_options (read provider repo settings), merge_payload (build provider-specific merge body) and a Tauri command get_integration_review_merge_options. Includes unit tests for behavior.
- Wire merge module into integrations.rs and pass an optional merge_method into provider-specific review action functions (GitHub, GitLab, Gitea, Azure DevOps). run_integration_review_action now accepts an optional merge_method, validates it early, and includes provider-specific merge payloads when performing a merge.
- Export the new command in src-tauri/src/main.rs so the frontend can request merge options.

Frontend changes to support selecting a merge method before merging:

- ConfirmDialog.svelte: add SelectMenu support and a select field to confirm requests.
- ReviewCenter.svelte: fetch integration merge options, show a merge-method selector in the merge confirmation, and pass the chosen method to the review action.
- Update types and git bindings to surface IntegrationMergeOptions / IntegrationMergeMethod and the getIntegrationReviewMergeOptions call (git.ts / types.ts changes staged).

Effect: users can pick a merge method appropriate to the provider/project; the integration layer generates the correct API payload per provider. Tests added for merge logic.

src/lib/git.ts

@@ -19,6 +19,8 @@ import type {
   GitIntegrationRepository,
   IntegrationReviewRequest,
   IntegrationReviewAction,
+  IntegrationMergeMethod,
+  IntegrationMergeOptions,
   GitLfsStatus,
   GitRepositoryFile,
   GitRemote,
@@ -66,8 +68,12 @@ export function listIntegrationReviewRequests(provider: GitIntegrationProvider,
   return invoke<IntegrationReviewRequest[]>("list_integration_review_requests", { provider, baseUrl, username, token, state });
 }
 
-export function runIntegrationReviewAction(provider: GitIntegrationProvider, baseUrl: string, username: string, token: string, request: IntegrationReviewRequest, action: IntegrationReviewAction): Promise<void> {
-  return invoke<void>("run_integration_review_action", { provider, baseUrl, username, token, repositoryId: request.repositoryId, repositoryName: request.repositoryName, number: request.number, action });
+export function getIntegrationReviewMergeOptions(provider: GitIntegrationProvider, baseUrl: string, token: string, request: IntegrationReviewRequest): Promise<IntegrationMergeOptions> {
+  return invoke("get_integration_review_merge_options", { provider, baseUrl, token, repositoryId: request.repositoryId, repositoryName: request.repositoryName });
+}
+
+export function runIntegrationReviewAction(provider: GitIntegrationProvider, baseUrl: string, username: string, token: string, request: IntegrationReviewRequest, action: IntegrationReviewAction, mergeMethod?: IntegrationMergeMethod): Promise<void> {
+  return invoke<void>("run_integration_review_action", { provider, baseUrl, username, token, repositoryId: request.repositoryId, repositoryName: request.repositoryName, number: request.number, action, mergeMethod });
 }
 
 export function getIntegrationReviewDetails(provider: GitIntegrationProvider, baseUrl: string, username: string, token: string, request: IntegrationReviewRequest): Promise<IntegrationReviewRequest> {