feat(model): resolve action scope from the selected row kind

ActionScope is what an action is about to touch, resolved from the selection in
one place so no call site reinvents the mapping. A thread root contributes the
whole thread, a message row contributes one message, and messageCount is what
the status bar reports.

The count comes from totalCount, not from the loaded children. A thread that was
never expanded still has all of its messages, and counting only the rows that
happen to be on screen would understate what the action does: mutation-checked,
and the wrong version reports 1 message where 7 are about to be tagged.

A mixed selection is honoured as given rather than escalated to thread scope or
narrowed to message scope. Silently widening it would defeat the reason the
scope is shown at all.

1 files changed, 26 insertions, 0 deletions

diff --git a/src/types.h b/src/types.h
index ef822b9..d04670e 100644
--- a/src/types.h
+++ b/src/types.h
@@ -94,6 +94,32 @@ struct MessageNode
     }
 };
 
+/// What an action is about to touch, resolved from the selection.
+///
+/// Exists because the thread list holds two kinds of row since item 20, so a
+/// keypress alone no longer says whether it hit one message or seven. Actions
+/// take one of these rather than a bare list of thread ids, and the status bar
+/// reports it: this project's answer to that ambiguity is to make the scope
+/// visible, not to add a confirmation dialog. See CLAUDE.md on why.
+struct ActionScope
+{
+    QStringList threadIds;   ///< Whole threads to act on.
+    QStringList messageIds;  ///< Individual messages to act on.
+
+    /// Messages the action will touch in total, for the status bar. A whole
+    /// thread contributes all of its messages, a message row contributes one.
+    int messageCount = 0;
+
+    /// True when any whole thread is in scope, which drives the
+    /// "(whole thread)" suffix in the status bar.
+    bool wholeThread = false;
+
+    bool isEmpty() const
+    {
+        return threadIds.isEmpty() && messageIds.isEmpty();
+    }
+};
+
 /// One tag mutation, kept so it can be inverted for undo.
 struct TagChange
 {