calendar: add category-group links, fix iCal share URL, remove legend

- Link categories to user groups via new zielgruppen column on
  veranstaltung_kategorien (migration 017), editable in the category
  management UI with group checkboxes and chip display
- Fix broken iCal share link by adding ICAL_BASE_URL to docker-compose
  and falling back to CORS_ORIGIN when ICAL_BASE_URL is unset
- Remove the colored-dot legend footer from the month calendar view

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/Kalender.tsx

@@ -435,29 +435,6 @@ function MonthCalendar({
           );
         })}
       </Box>
-
-      {/* Legend */}
-      <Box sx={{ display: 'flex', flexWrap: 'wrap', gap: 1.5, mt: 2 }}>
-        {Object.entries(TYP_DOT_COLOR).map(([typ, color]) => (
-          <Box key={typ} sx={{ display: 'flex', alignItems: 'center', gap: 0.5 }}>
-            <Box sx={{ width: 8, height: 8, borderRadius: '50%', bgcolor: color }} />
-            <Typography variant="caption" color="text.secondary">{typ}</Typography>
-          </Box>
-        ))}
-        <Box sx={{ display: 'flex', alignItems: 'center', gap: 0.5 }}>
-          <Box
-            sx={{
-              width: 8, height: 8, borderRadius: '50%',
-              bgcolor: 'warning.main', border: '1.5px solid', borderColor: 'warning.dark',
-            }}
-          />
-          <Typography variant="caption" color="text.secondary">Pflichtveranstaltung</Typography>
-        </Box>
-        <Box sx={{ display: 'flex', alignItems: 'center', gap: 0.5 }}>
-          <Box sx={{ width: 8, height: 8, borderRadius: '50%', bgcolor: '#1976d2' }} />
-          <Typography variant="caption" color="text.secondary">Veranstaltung</Typography>
-        </Box>
-      </Box>
     </Box>
   );
 }