refactor(file-editor): use single shared instance across all tabs

[xlyk]

Summary

• Previously each tab maintained its own file editor state (open files, recent files, editor settings)
• Now the file editor uses a single global state shared across all tabs
• Files remain open when switching between main app tabs

Changes

• Flatten sessions map to single global state in file-editor-sidebar.ts store
• Remove sessionId parameters from useFileEditorSidebar hook
• Remove sessionId prop from FileEditorSidebarPanel component
• Update consumers (FilePathLink, StaticTerminalOutput, Markdown) to use simplified hook

Test plan

• Open file editor panel and open a file
• Switch to a different tab
• Verify file is still open in editor panel
• Verify editor settings (vim mode, wrap, etc.) persist across tab switches