feat(jira-knowledge): add contextual help tooltips and inline validation for Project Mapping

- Add QToolButton help icon (?) next to Project Mapping label
- Implement comprehensive tooltip explaining Project ID and Jira Key concepts
- Add inline validation to detect common mistake: entering issue keys (ABC-123) instead of project keys (ABC)
- Add 13 new i18n keys for help text in English, Vietnamese, and Japanese
- Add 15 comprehensive UX tests covering icon presence, tooltip content, validation logic, and accessibility
- Update jira-knowledge-guide.md with help icon reference and setup instructions
- Fix missing _on_paste method that was causing AttributeError

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/jira-knowledge-guide.md

@@ -34,6 +34,10 @@ Jira Project Knowledge enables Cowork to index Jira issues as searchable project
 1.  In the same Jira dialog, check **Enable Jira Project Knowledge**.
 2.  Enter **Project Mapping** in the format `cowork_project_id:JIRA_PROJECT_KEY`.
     *   Example: `proj-alpha:ALPHA, proj-beta:BETA`
+    *   Click the **ⓘ** icon next to "Project Mapping" for detailed help on:
+        *   **Project ID**: The Cowork project identifier (e.g., `cowork-local`). Find it in your current Cowork project settings.
+        *   **Jira Key**: The Jira project key (e.g., `ALPHA` from issue `ALPHA-123`). Open any Jira issue to find it.
+    *   **Common mistake**: Do not enter issue keys like `ABC-123`. Only enter the project key part `ABC`.
 3.  Click **Save**.
 
 ### 3.3 Initial Sync