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/components/ConfirmDialog.svelte

@@ -5,6 +5,7 @@
    * untranslated, event-blocking browser dialog.
    */
   import { AlertTriangle, Check, LoaderCircle, Trash2, X } from "@lucide/svelte";
+  import SelectMenu from "./SelectMenu.svelte";
   import { t } from "../i18n.svelte";
 
   export interface ConfirmRequest {
@@ -25,6 +26,7 @@
     input?: { label: string; placeholder?: string; value?: string; optional?: boolean };
     /** Destructive actions get the red confirm button and warning icon. */
     danger?: boolean;
+    select?: { label: string; value: string; options: { value: string; label: string }[] };
   }
 
   interface Props {
@@ -47,13 +49,13 @@
   let value = $state("");
   let inputElement = $state<HTMLInputElement | null>(null);
   let missingInput = $derived(Boolean(request.input) && request.input?.optional !== true && value.trim().length === 0);
-  let blocked = $derived((Boolean(request.checkbox?.required) && !checked) || missingInput);
+  let blocked = $derived((Boolean(request.checkbox?.required) && !checked) || missingInput || (Boolean(request.select) && !request.select?.options.some(option => option.value === value)));
 
   $effect(() => {
     // Start from the defaults again whenever a different confirmation is shown.
     request.title;
     checked = request.checkbox?.defaultChecked ?? false;
-    value = request.input?.value ?? "";
+    value = request.select?.value ?? request.input?.value ?? "";
   });
 
   $effect(() => {
@@ -144,6 +146,13 @@
           </label>
         {/if}
 
+        {#if request.select}
+          <div class="confirm-input">
+            <span>{request.select.label}</span>
+            <SelectMenu {value} options={request.select.options} ariaLabel={request.select.label} disabled={isBusy} onChange={(selected) => value = selected} />
+          </div>
+        {/if}
+
         {#if request.checkbox}
           <label class="confirm-check">
             <input type="checkbox" bind:checked disabled={isBusy} />