Add more desk exercises

[turqoisehex]

Summary

Screen Break has a list of desk exercises in `DESK_EXERCISES` (line ~348) that are shown during micro-pauses. Adding more exercises with clear instructions would give users more variety and better stretching coverage.

Acceptance Criteria

• Add 5-10 new exercises targeting different body areas (wrists, shoulders, back, legs, neck)
• Each exercise has: name, duration (seconds), and steps list (strings)
• Exercises display correctly in the micro-pause overlay
• Steps are concise and doable at a desk

Relevant Code

• `DESK_EXERCISES` list: `screen_break.py` line ~348
• Exercise display during micro-pause: line ~2193

Notes

Each exercise is a dict with "name", "duration", and "steps" (list of instruction strings). Just append new entries to the list. The overlay randomly selects one exercise per micro-pause.