v6.4.2 — Cleaner daily study, floating feature removed

Remove the floating focus feature entirely and simplify daily study.

• Delete the timer, overlay, picture-in-picture windows and completion popups.
• Show today's scheduled subjects as clean task rows with completion/undo and expandable resources.
• Preserve saved progress, calendar rollover, pending work and tomorrow's preview.

Validation: ESLint clean; 93 unit tests, 553 planner checks and 5 browser scenarios pass. Mobile/desktop completion, undo, AI tutor links, persistence and offline reload verified.