Make the PyQt calendar Today action obvious and verified

[ZFordDev]

Context

The PyQt calendar already creates a Today button and connects it to go_to_today, but the control is not covered by a dedicated navigation test and is not sufficiently discoverable in normal use.

Scope

• Review the placement and visual hierarchy of the existing Today control in month, week, and day views.
• Give the control an explicit accessible name and useful tooltip.
• Verify it selects the current local date, changes the displayed month when required, and refreshes the active agenda/grid.
• Keep this confined to the flagship PyQt interface.

Acceptance criteria

• Today remains visible and understandable after navigating months or weeks away.
• Activating it returns month, week, and day views to the current local date.
• Keyboard and screen-reader users can identify and activate it.
• Automated PyQt tests cover navigation away and return to today.
• The desktop guide mentions the control if its placement or interaction changes.