feat(integrations): add assignee/assignment support

Add a new integrations/assignees backend (src-tauri/src/integrations/assignees.rs)
and expose commands to list, read and set assignees:
- list_integration_assignees
- get_integration_assignees
- set_integration_assignees

Introduce IntegrationAssignee and AssignmentTarget types and provider-specific
URL/payload logic (GitHub, Gitea, GitLab / self-hosted, Azure DevOps). The code
handles pagination, provider quirks (Gitea legacy fields, GitLab assignee_ids,
Azure reviewer vs work-item differences) and validates/verifies assignments.
Unit tests cover routing and payload behavior.

Add UI components AssigneePicker.svelte and AssignmentEditor.svelte and update
CreateIssueDialog, CreateReviewDialog, IssueCenter, ReviewCenter, git types and
git.ts to use the new assignment functionality.

src-tauri/src/main.rs

@@ -37,6 +37,7 @@ use git::{
 use integrations::{
     create_integration_review_request, list_integration_repository_branches,
     add_integration_review_comment, get_integration_review_details, list_integration_repositories, list_integration_review_requests, open_in_browser,
+    list_integration_assignees, get_integration_assignees, set_integration_assignees,
     create_integration_issue, list_azure_issue_projects, list_azure_issue_types,
     run_integration_review_action, get_integration_review_merge_options, list_integration_issues, get_integration_board, list_integration_boards, move_integration_board_card, list_integration_issue_comments, add_integration_issue_comment, close_integration_issue, list_azure_issue_states, set_azure_issue_state,
 };
@@ -456,6 +457,9 @@ async fn main() {
             move_integration_board_card,
             list_integration_issue_comments,
             add_integration_issue_comment,
+            list_integration_assignees,
+            get_integration_assignees,
+            set_integration_assignees,
             create_integration_issue,
             list_azure_issue_projects,
             list_azure_issue_types,