feat: EmailTreeModel grouping modes + MailListView integration

- EmailTreeModel: 4 grouping modes
  * NoGrouping - flat list
  * GroupByDate - chronological groups (Hoy, Ayer, Esta semana...)
  * GroupByThread - conversations with expandable threads (ThreadId from References/In-Reply-To/Message-ID)
  * GroupBySender - groups by sender email/name

- EmailTreeModel thread support:
  * ThreadItem type with expand/collapse
  * ThreadId extraction via ThreadUtils (References > In-Reply-To > Message-ID)
  * Double-click thread node toggles expansion
  * Threads sorted by newest email first, emails within thread oldest first

- EmailTreeModel sender grouping:
  * Groups by sender name/email
  * Senders sorted by newest email first
  * Emails within group newest first

- MailListView: Added 'Agrupar por remitente' to group-by combo
- MailListView: Double-click thread node toggles expansion (not open mail)
- ThreadUtils: parseReferencesHeader, extractThreadId, generateThreadId

src/ui/models/EmailTreeModel.h

@@ -25,7 +25,10 @@ public:
         IsGroupRole = Qt::UserRole + 2,
         GroupNameRole = Qt::UserRole + 3,
         GroupIndexRole = Qt::UserRole + 4,
-        MailIndexRole = Qt::UserRole + 5
+        MailIndexRole = Qt::UserRole + 5,
+        IsThreadRole = Qt::UserRole + 6,
+        ThreadIdRole = Qt::UserRole + 7,
+        IsThreadExpandedRole = Qt::UserRole + 8
     };
 
     explicit EmailTreeModel(QObject *parent = nullptr);
@@ -47,20 +50,28 @@ public:
     // Grouping
     enum GroupMode {
         NoGrouping = 0,
-        GroupByDate = 1
+        GroupByDate = 1,
+        GroupByThread = 2,
+        GroupBySender = 3
     };
     void setGroupMode(GroupMode mode);
     GroupMode groupMode() const { return m_groupMode; }
 
+    // Thread expansion
+    void setThreadExpanded(const QString& threadId, bool expanded);
+    bool isThreadExpanded(const QString& threadId) const;
+
 private:
     struct TreeItem {
-        enum Type { Group, Mail } type;
+        enum Type { Group, Mail, Thread } type;
         QString groupName;          // for Group type
+        QString threadId;           // for Thread type
         int groupIndex = -1;        // for Group type
         int mailIndex = -1;         // for Mail type (index in m_emails)
         QVector<TreeItem*> children;
         TreeItem* parent = nullptr;
         int row = -1;
+        bool expanded = true;       // for Thread type
 
         TreeItem(Type t, TreeItem* p = nullptr) : type(t), parent(p) {}
         ~TreeItem() { qDeleteAll(children); }
@@ -69,10 +80,15 @@ private:
     GroupMode m_groupMode = NoGrouping;
     QVector<MailItem> m_emails;
     TreeItem* m_rootItem = nullptr;
+    QSet<QString> m_expandedThreads;
 
     void setupModelData();
     void buildGroups();
+    void buildThreads();
+    void buildSenderGroups();
     QString groupNameForDate(const QDateTime &date) const;
     int groupIndexForDate(const QDateTime &date) const;
     void deleteTree(TreeItem* item);
+    void buildThreadTree();
+    QString extractThreadId(const MailItem& mail) const;
 };