Insert a working repeat block instead of its description

The placeholder chooser showed repeat blocks as "{{#claims}} … {{/claims}}" and
inserted exactly that on double-click -- the ellipsis is a label, so the saved
template rendered a literal "…" per claim instead of the claim.

Chooser rows are data now: each carries the text it reads as and, separately,
the snippet it inserts. A block contributes a complete, ready-to-edit block with
a sample line built from its own placeholders, and lands on a line of its own
when the cursor sits behind existing text.

Covered from both sides: the snippets are checked against the template validator
and renderer, and a UI test drives the real dialog -- insert, save, send -- and
asserts the mail carries actual claim lines. The Tk tests share one root (the
icon library binds its images to the first one) and skip without a display.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_mail_templates.py

@@ -6,6 +6,7 @@ from ccma.domain.mail_templates import (
     MailTemplateError,
     default_mail_template,
     parse_mail_template,
+    placeholder_entries,
     render_mail_template,
     serialize_mail_template,
     validate_mail_template,
@@ -103,3 +104,53 @@ def test_saving_a_broken_template_is_refused(tmp_path) -> None:
         repository.save_mail_template("reminder", subject="Mahnung", body="Hallo {{member.nonsense}}")
     with pytest.raises(RepositoryError, match="Betreff"):
         repository.save_mail_template("reminder", subject="   ", body="Hallo")
+
+
+def test_block_rows_insert_a_working_block_not_its_description() -> None:
+    entries = placeholder_entries("welcome")
+    block = next(entry for entry in entries if entry.label.startswith("{{#claims}}"))
+
+    # The label may abbreviate the block, the inserted snippet may not.
+    assert block.label == "{{#claims}} … {{/claims}}"
+    assert block.snippet.startswith("{{#claims}}\n")
+    assert block.snippet.rstrip().endswith("{{/claims}}")
+    assert "{{claim.description}}" in block.snippet
+    assert "…" not in block.snippet
+    assert all("…" not in entry.snippet for entry in entries)
+
+
+def test_inserted_block_validates_and_renders_one_line_per_claim() -> None:
+    block = next(
+        entry for entry in placeholder_entries("welcome") if entry.label.startswith("{{#claims}}")
+    )
+    body = f"Hallo {{{{member.first_name}}}},\n\n{block.snippet}\nSumme: {{{{claims.total}}}}"
+
+    validate_mail_template("welcome", "Betreff", body)
+    rendered = render_mail_template(
+        MailTemplate("Betreff", body),
+        {"member.first_name": "Ada", "claims.total": "75.00"},
+        {
+            "claims": [
+                {
+                    "claim.description": "Aufnahmegebühr",
+                    "claim.due_date": "17.09.2026",
+                    "claim.balance": "15.00",
+                },
+                {
+                    "claim.description": "Mitgliedsbeitrag 2. Halbjahr 2026",
+                    "claim.due_date": "17.09.2026",
+                    "claim.balance": "60.00",
+                },
+            ]
+        },
+    )
+
+    assert "Aufnahmegebühr (fällig 17.09.2026): 15.00 Euro" in rendered.body
+    assert "Mitgliedsbeitrag 2. Halbjahr 2026 (fällig 17.09.2026): 60.00 Euro" in rendered.body
+    assert "…" not in rendered.body
+
+
+def test_every_block_snippet_is_valid_in_its_own_template() -> None:
+    for spec in MAIL_TEMPLATES:
+        for block in spec.blocks:
+            validate_mail_template(spec.key, "Betreff", block.snippet)