🔧 Mobile-friendly calendar with agenda view

[fusion94]

Summary

The 7-column month grid in src/pages/CalendarPage.jsx is unusable on phones — cells become unreadable and event chips overflow. Below the sm MUI breakpoint, switch to an agenda view: a vertical list of upcoming events grouped by day.

Expected Behavior

• On xs (mobile), /app/calendar shows an agenda: today + next ~30 days, each day with its events as cards.
• On sm+ (tablet/desktop), unchanged month grid.
• Agenda still supports create/edit/delete event flows.
• "Subscribe" / "Export ICS" buttons remain accessible.

Proposed Approach

1. Use useMediaQuery(theme.breakpoints.down('sm')) to gate the rendering branch.
2. Extract the existing event list/dialog rendering into a shared component used by both views; agenda just removes the grid wrapper.
3. Add date jump controls (next/prev week, "today" shortcut) at the top of the agenda.

Acceptance Criteria

• At < 600px width, calendar renders as a scrollable vertical agenda.
• At >= 600px, renders as the existing month grid.
• Event create/edit/delete works in both views.
• Multi-day events render sensibly in agenda (one entry per day they cover, or a single row spanning).
• Both views work in both themes.