refactor: replace boolean view state with single activeView string

- Replace 10 boolean useState declarations with single activeView state
- Simplify goToDashboard from 12 lines to 1 line
- Simplify handleLogout from 10 state resets to 1
- Replace nested ternary render chain with flat conditional rendering
- AdminPanel.jsx reduced from 761 to 704 lines (total reduction from ~1300: -46%)

Author: bartekmp