Participants, context menus, budget rename, file types, UI polish

- Assignment participants: toggle who joins each activity
  - Chips with hover-to-remove (strikethrough effect)
  - Add button with dropdown for available members
  - Avatars in day plan sidebar
  - Side-by-side with reservation in place inspector
- Right-click context menus for places, notes in day plan + places list
- Budget categories can now be renamed (pencil icon inline edit)
- Admin: configurable allowed file types (stored in app_settings)
- File manager shows allowed types dynamically
- Hotel picker: select place + save button (no auto-close)
- Edit pencil opens full hotel popup with all options
- Place inspector: opening hours + files side by side on desktop
- Address clamped to 2 lines, coordinates hidden on mobile
- Category shows icon only on mobile
- Rating hidden on mobile in place inspector
- Time validation: "10" becomes "10:00"
- Climate weather: full hourly data from archive API
- CustomSelect: grouped headers support (isHeader)
- Various responsive fixes

client/src/components/Planner/DayPlanSidebar.jsx

@@ -752,6 +752,23 @@ export default function DayPlanSidebar({
                                   </div>
                                 )
                               })()}
+                              {assignment.participants?.length > 0 && (
+                                <div style={{ marginTop: 3, display: 'flex', alignItems: 'center', gap: -4 }}>
+                                  {assignment.participants.slice(0, 5).map((p, pi) => (
+                                    <div key={p.user_id} style={{
+                                      width: 16, height: 16, borderRadius: '50%', background: 'var(--bg-tertiary)', border: '1.5px solid var(--bg-card)',
+                                      display: 'flex', alignItems: 'center', justifyContent: 'center', fontSize: 7, fontWeight: 700, color: 'var(--text-muted)',
+                                      marginLeft: pi > 0 ? -4 : 0, flexShrink: 0,
+                                      overflow: 'hidden',
+                                    }}>
+                                      {p.avatar ? <img src={p.avatar} style={{ width: '100%', height: '100%', objectFit: 'cover' }} /> : p.username?.[0]?.toUpperCase()}
+                                    </div>
+                                  ))}
+                                  {assignment.participants.length > 5 && (
+                                    <span style={{ fontSize: 8, color: 'var(--text-faint)', marginLeft: 2 }}>+{assignment.participants.length - 5}</span>
+                                  )}
+                                </div>
+                              )}
                             </div>
                             <div className="reorder-buttons" style={{ flexShrink: 0, display: 'flex', gap: 1, opacity: isHovered ? 1 : undefined, transition: 'opacity 0.15s' }}>
                               <button onClick={moveUp} disabled={placeIdx === 0} style={{ background: 'none', border: 'none', padding: '1px 2px', cursor: placeIdx === 0 ? 'default' : 'pointer', color: placeIdx === 0 ? 'var(--border-primary)' : 'var(--text-faint)', display: 'flex', lineHeight: 1 }}>