Add hover tooltips and Switch Agent header to agent selector

[ForisKuang]

Summary

• Add showSwitchAgent config flag to model specs for enabling the switch agent feature
• Add "Switch Agent" pill button on the landing page to quickly switch between agents
• Implement agent switching logic using getConvoSwitchLogic to handle conversation and preset transitions
• Extract "Learn more" link from greeting HTML and render side by side with "Switch Agent" button
• Add hover tooltips with agent descriptions and "Switch Agent" header to the agent selector in the nav bar
• Add com_ui_switch_agent translation key

Test plan

• Verify landing page shows "Learn more" (grey) and "Switch Agent" (themed) buttons side by side
• Verify clicking "Switch Agent" switches to the other agent and updates the landing page greeting, icon, and description
• Verify agent selector in header shows tooltip with agent description on hover
• Verify showSwitchAgent: true in librechat.yaml enables the button for each spec
• Verify switching agents correctly updates the conversation preset and endpoint

Screenshots

Test Plan

• Hover over each agent button and verify tooltip shows agent description
• Verify "Switch Agent" label appears before the agent buttons
• Click agent buttons and verify switching still works correctly
• Verify tooltip falls back to label/name if no description is set