fix(dashboard): sidebar theming, calendar reactivity, and booking filters

Fix multiple dashboard and UI issues:

Frontend fixes:
- Fix sidebar remaining dark on light theme (add proper light/dark CSS variables)
- Fix DashboardCalendar blank/not showing events (use watch + calendar API instead of computed options)
- Fix upcoming bookings to include active and recent past (last 7 days) bookings
- Improve sidebar collapsed state UX (stack footer buttons vertically, full width)

Details:
- theme.css: Add light sidebar colors (white bg) for :root, keep dark colors for [data-theme="dark"]
- DashboardCalendar: Add watch on events, use calendarRef to update events via removeAllEvents/addEventSource
- Dashboard: Change upcoming filter from "startDate >= now" to "endDate >= 7 days ago"
- AppSidebar: Stack footer-actions vertically when collapsed for better visibility

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/components/AppSidebar.vue

@@ -148,6 +148,16 @@ const handleLogout = () => {
   width: var(--sidebar-collapsed-width);
 }
 
+.sidebar.collapsed .footer-actions {
+  flex-direction: column;
+  gap: 0.5rem;
+}
+
+.sidebar.collapsed .footer-btn {
+  width: 100%;
+  justify-content: center;
+}
+
 /* Header */
 .sidebar-header {
   display: flex;