feat(macos): Cursor usage indicator + Cursor/Claude switcher

[fenix1851]

Summary

• Add a macOS rumps indicator for Cursor plan usage (totalPercentUsed / Auto / API / on-demand) using the local session token from state.vscdb.
• Add a unified menu bar (macos/menubar.py) with a drop-down radio switcher between Cursor and Claude Code; last source is remembered under ~/.config/limitschecker/source.
• Document env vars and the Cursor pool semantics (avoid treating includedSpend/limit as the main bar).

Test plan

• python3 -m venv .venv && .venv/bin/pip install rumps
• .venv/bin/python macos/menubar.py with Cursor signed in — panel shows Cursor · Total … · Auto … · API …
• Switch to Claude Code in the menu — panel prefix becomes Claude · … (needs Claude Code logged in / Keychain token)
• Quit and relaunch — last selected source is restored
• Optional: .venv/bin/python macos/cursor_limitschecker.py still works standalone