feat(integrations): add labels integration and expose shared IntegrationApi

Add a new labels integration (src-tauri/src/integrations/labels.rs) that implements listing,
reading and updating issue labels for multiple providers (GitHub, Gitea, GitLab and Azure
DevOps). The new module provides Tauri commands:
- list_integration_labels
- get_integration_issue_labels
- set_integration_issue_labels

Refactor assignees integration to make core HTTP helpers reusable:
- Rename AssignmentApi -> IntegrationApi and make its fields/methods pub(super).
- Make helper functions api_url, repository_parts and target_url pub(super) so labels.rs
  can construct and call provider endpoints.
- Adjust wording of a few error messages (e.g. "Assignment request..." -> "Integration request...")
  and genericize some page/result error text.

Export the new labels module from integrations.rs and relax test helper visibility
(pub(crate)) so the labels tests can reuse the existing fixture utilities.

This commit introduces provider-specific parsing and payload logic for labels and
reuses the shared IntegrationApi to perform authenticated requests.

src-tauri/src/integrations.rs

@@ -1,3 +1,5 @@
+mod labels;
+pub use labels::*;
 mod assignees;
 pub use assignees::*;
 mod cleanup;