Improve 2D plot UX, external API routing, and state handling

[elainefan331]

Description:
This PR improves the NeuroJSON data preview workflow by enhancing the 2D plotting experience, refining external API handling through the Express backend, and fixing several UI/state management issues.

Changes:

2D Plot & Visualization Improvements

• Added axis labels to 2D plots (Sample / a.u.)
• Added interaction tips for 2D plot usability
• Display source path and index above the 2D chart panel

API & Backend Routing

• Refactored frontend baseURL handling to route through the Express server
• Mapped external API calls through Express to avoid firewall/CORS-related issues
• Removed legacy frontend CORS proxy logic

State & Data Handling Fixes

• Fixed markdown fence stripping from BIDSPlan YAML content before saving
• Fixed stale document and revision state persistence when navigating between pages/views