Enhance multi-file workflow — better tab management, file switching, and unsaved changes indicator #2

Description

The current multi-file experience needs improvement in tab management, quick file switching, and visual feedback for unsaved changes.

Current Behavior

• Tab management is basic
• No clear visual indicator when a file has unsaved changes
• Switching between multiple files in a workspace can feel slow or confusing
• No "close all tabs" or better organization options

Expected Behavior

• Clear "*" or dot indicator on tabs with unsaved changes (like VS Code)
• Smooth and fast file switching
• Better tab reordering and "pin tab" functionality (nice-to-have)
• Keyboard shortcuts for quick navigation (Ctrl/Cmd + Tab, Ctrl/Cmd + W to close, etc.)
• Option to show file path in tab when there are duplicate filenames

Why This Matters

LabSTX's biggest differentiator is proper multi-file Clarinet project support. Improving the multi-file UX will make the tool feel much more professional and productive compared to single-contract playgrounds like Hiro Sandbox.

Acceptance Criteria

• Unsaved changes indicator added to tabs
• Smooth tab switching and management
• At least 3 keyboard shortcuts implemented for file navigation
• Works well with 5+ open files in a workspace