feature/VSN-2860 - resolve 15 UI bugs across ticket management module && Fix unread counter, duplicate message bubble, mini back navigation in chat.

Tickets :

• Fix search firing on every keystroke (debounce + instant client-side filter)
• Fix client-side filter using debouncedSearch instead of raw search (1-letter bug)
• Add "Sélectionner tout" to team MultiSelect in CreateTicketModal
• Add dirty-check confirmation banner on modal close with unsaved changes
• Fix KPI auto-refresh in GlobalTicketsPage (refreshAll pattern)
• Fix KPI/Kanban count mismatch in GlobalTicketsPage (use filtered tickets.length)
• Fix team selection reset on page refresh (sessionStorage persistence)
• Add assignedTo filter dropdown in GlobalTicketsPage advanced filters
• Fix refresh button in GlobalTicketsPage (missing label + calling refreshAll)
• Add inline save confirmation banner in TicketPanel after edit
• Add "Assigné à moi" as first option in collaborator list (sort self first)
• Auto-assign ticket to current user when assignedTo left empty
• Make assignedTo field optional in ticket schema + auto-select self on load
• Fix board onStatusChange to call refreshAll (KPIs stay in sync after drag)
• Fix pagination filtered count display (shows "X filtrés sur Y" when active)

Chat :

• Fixed a stale closure bug in useMessages.ts where userId = null at listener registration caused sent messages to render as both sent and received.
• Fixed the unread counter in useConversation.ts by reading activeConversation directly from the Zustand store instead of a potentially stale ref.
• Fixed mini-mode back navigation in ChatWidget.tsx by also clearing activeConversation on back, preventing the conversation panel from re-mounting immediately.

Closes VSN-2860