Merge branch 'rule-builder': a row builder for the tagging rules

Item 76 replaces the four free-text fields with a row builder: field and
operator dropdowns per condition, +/- to add and remove them, a match
all/any choice, and a separate "but not" block. The stored format does
not change, so mailctl needs no edit. The query string stays
authoritative and remains visible, and a rule the builder cannot
represent opens in a text mode every rule carries.

Along the way, items 75, 77 and 80, and a data-loss defect released in
0.16.0 (item 79): opening the dialog and pressing Save destroyed the
first rule with nothing edited. That one damaged a real rule in the
user's own file, which was repaired by hand.

Four defects in this work were found by hand rather than by the suite,
and each is recorded where it was missed: a lost note, a one-way text
mode toggle, a geometry save on a path neither button takes, and a rule
list squeezed to one row by a long rule. Two Qt traps and one about the
user's compositor went into CLAUDE.md.

1 files changed, 26 insertions, 0 deletions

diff --git a/src/mainwindow.h b/src/mainwindow.h
index adf63b0..18fbbb4 100644
--- a/src/mainwindow.h
+++ b/src/mainwindow.h
@@ -185,6 +185,27 @@ public:
     /// The generation the next counts reply must carry to be accepted.
     quint64 countsGenerationForTesting() const { return m_countsGeneration; }
 
+    /// The query bar's text, and the account the selector is scoped to
+    /// (empty for "All accounts"). Both are what a rule preview writes: the
+    /// bar so the user can see and edit what ran, and the selector because
+    /// runQuery() wraps the text in the selected account's scope, which would
+    /// double-scope a rule query that already names its own path.
+    QString queryTextForTesting() const;
+    QString selectedAccountForTesting() const;
+
+    /// Scopes the view to one account, as choosing it in the selector does.
+    /// A test for the rule preview needs this: with no account selected the
+    /// box already sits at "All accounts", so asserting that a preview leaves
+    /// it there passes whether or not the preview clears it.
+    void selectAccountForTesting(const QString &key);
+
+    /// Runs a rule preview without the dialog, which the offscreen platform
+    /// cannot click a button in.
+    void previewRuleQueryForTesting(const QString &query)
+    {
+        onRulePreviewRequested(query);
+    }
+
 protected:
     void closeEvent(QCloseEvent *event) override;
 
@@ -293,6 +314,11 @@ private slots:
     /// Runs a query the user clicked on the placeholder pane.
     void onPlaceholderQueryRequested(const QString &query);
 
+    /// Runs one tagging rule's query in the thread list, so the user can see
+    /// which mail it collects. The rules dialog stays open; the point is to
+    /// compare the rule against its results.
+    void onRulePreviewRequested(const QString &query);
+
     /// Opens the auto-tagging rules editor, or raises the one already open.
     void showTagRulesDialog();