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

build/CMakeFiles/wino-mail-qt_autogen.dir/AutogenInfo.json

@@ -655,6 +655,12 @@
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+			"MU",
+			"VSCBVMNR7M/moc_threadutils.cpp",
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@@ -1065,6 +1071,11 @@
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+			"/mnt/c/Users/javie/Documents/Codex/2026-08-11/revisar-2/wino-mail-dtkqt/src/utils/threadutils.cpp",
+			"MU",
+			null
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 		[
 			"/mnt/c/Users/javie/Documents/Codex/2026-08-11/revisar-2/wino-mail-dtkqt/thirdparty/tags/src/config.cpp",
 			"MU",