feat: add cross-platform embedded terminal

[danyalahmed1995]

Summary

Adds an optional cross-platform embedded terminal to EXT, available from Settings and toggleable with a keyboard shortcut.

Changes

• Added an embedded terminal panel below the editor/preview area.

• Added Settings toggles for:

  • Enable embedded terminal
  • Auto-sync directory to active file

• Added platform-aware shell support:

  • Windows: pwsh / PowerShell
  • macOS: default shell / zsh
  • Linux: default shell / bash / sh

• Added PTY-backed terminal process handling.

• Added terminal controls:

  • Sync CWD
  • Clear
  • Restart
  • Close

• Added Ctrl + \`` / Cmd + `` shortcut to toggle the terminal.

• Added automatic cwd syncing across files and workspaces.

• Added manual cwd syncing when auto-sync is disabled.

• Styled the terminal to match EXT themes.

• Fixed terminal blanking after clear and tab switching.

• Fixed visual terminal artifacts such as the stray black vertical line.

• Prevented unnecessary terminal restarts when switching files.

• Added process cleanup when terminal is disabled or EXT fully quits.

Testing

• Verified terminal can be enabled and disabled from Settings.
• Verified terminal opens in the active file directory.
• Verified cwd sync works across different files and workspaces.
• Verified manual Sync CWD works when auto-sync is disabled.
• Verified paths with spaces and non-C drive paths work on Windows.
• Verified clear keeps the terminal interactive.
• Verified terminal theme matches EXT Dark and Gruvbox Glass.
• Verified `Ctrl + `` toggles terminal visibility.
• Verified terminal does not restart unnecessarily on tab switch.
• Verified disabling terminal cleans up the shell process.
• Verified full quit does not leave orphaned terminal processes.
• Verified Editor, Split, and Preview layouts still work.