feat: manage state using external session file

[vjousse]

fixes #42

⚙️ TODO

• Use file management with the cli
• Create file when interacting with the gui
• Use file management to handle gui interaction

📖 How to test

Start a focus session with the label working:

echo "focus;working" > ~/.cache/pomodorolm_session

Stop the current focus session:

rm ~/.cache/pomodorolm_session

Reset/Restart the current session:

echo "focus;working" > ~/.cache/pomodorolm_session