[codex] Add explorer QoL basics

[parsakhaz]

What changed

This adds the missing basic Explorer interactions users expect from a code editor sidebar.

Highlights

• added a fuller Explorer context menu with create, rename, copy, cut, paste, duplicate, path copy, reveal, and trash actions
• added inline rename in the tree plus keyboard basics like F2, Delete, Cmd/Ctrl+C, Cmd/Ctrl+X, and Cmd/Ctrl+V
• added drag-and-drop move support for tree items and target-aware external file drops into folders or root
• added main-process file operations for rename, move, copy, duplicate, targeted binary writes, and trash-backed delete with fallback
• prevented the global Cmd/Ctrl+A and Cmd/Ctrl+D tab-cycling shortcuts from firing while an input is focused, so text fields in Explorer and browser tabs keep native behavior

Why

The existing Explorer could open and edit files, but it was missing the baseline quality-of-life file management behaviors users expect from VS Code-like Explorer UI.

Impact

Explorer panels now support routine file management without forcing users into the terminal or external file manager for basic operations.

Validation

• pnpm --filter frontend typecheck
• pnpm --filter main typecheck
• pre-commit checks passed in a clean worktree after linking the repo node_modules into that temporary checkout