fix(composer): an element the template cannot draw broke generation

Reproduced from the review comment. A `group` element carrying minWidth
generated:

    if width >= 64:  # breakpoint: 64px+ displays only
    # <nothing>

manager.py.j2 wraps each element in the breakpoint and blink blocks, but the
body comes from the per-type branches -- and a type with no branch contributes
nothing, so the wrapper opens a block with no statements. ast.parse then fails
and the caller is told only "Generated code has a syntax error: expected an
indented block ... line 49", naming a line of generated source they never see.

Two defences:

- _preprocess_elements drops types the template has no branch for, alongside
  the existing `section` skip. This is the root cause: those elements should
  never have reached the template.
- The branch chain ends in `{% else %}pass`, so a type added to the canvas
  before its drawing branch exists degrades to a no-op rather than a plugin
  that will not parse.

The review also cited dynamic_text with binding_source != 'config'. That one
does not reproduce -- the branch emits a draw_text regardless -- which is why
an earlier attempt to reproduce this found nothing.

_RENDERABLE_ELEMENT_TYPES has to stay in step with the template: a type listed
with no branch emits an empty block again, and a branch missing from the list
is silently dropped from every generated plugin. A test asserts the two sets
are equal rather than trusting them to be maintained together.

Tests: 12 new, covering group/unknown/section against breakpoint, blink and
both nested, plus the set-equality and fallback checks. 7 fail with both
defences reverted. 172 composer tests pass; full suite 3862 passed, the one
failure being test_install_lowmem (pre-existing, awaiting #492).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW

test/test_composer_empty_block.py

@@ -0,0 +1,80 @@
+"""An element the template cannot draw must not produce an empty `if` block.
+
+manager.py.j2 wraps each element in `if width >= N:` (breakpoint) and/or
+`if int(time.time() * 2) % 2:` (blink), and the body comes from the per-type
+branches. A type with no branch contributed nothing, so the wrapper opened a
+block with no statements in it. ast.parse in _generate_plugin_files then
+failed and the caller was told only:
+
+    Generated code has a syntax error: expected an indented block after
+    'if' statement on line 49
+
+which names a line of generated source the user never sees. Confirmed against
+the code before the fix with a `group` element carrying minWidth.
+
+Two defences, both covered here: _preprocess_elements drops types the template
+has no branch for, and the template emits a `pass` fallback so a type added to
+the canvas before its branch exists degrades to a no-op instead of a broken
+plugin.
+"""
+import re
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from web_interface.blueprints import composer as C  # noqa: E402
+
+TEMPLATE = (Path(__file__).resolve().parent.parent
+            / "web_interface/templates/v3/composer/manager.py.j2")
+
+BASE_META = {"id": "test-plugin", "name": "Clock", "author": "a",
+             "version": "1.0.0", "description": "d"}
+
+
+def generate(element):
+    return C._generate_plugin_files({
+        "metadata": BASE_META,
+        "elements": [element],
+        "dataModel": {"configVars": []},
+    })
+
+
+@pytest.mark.parametrize("wrapper", [
+    {"minWidth": 64},          # breakpoint block
+    {"blink": True},           # blink block
+    {"minWidth": 64, "blink": True},   # both, nested
+])
+@pytest.mark.parametrize("etype", ["group", "widget_9000", "section"])
+def test_undrawable_element_does_not_break_generation(etype, wrapper):
+    element = {"type": etype, "x": 0, "y": 0, "color": "#ffffff", **wrapper}
+    files = generate(element)          # must not raise ComposerInputError
+    assert "manager.py" in files
+
+
+def test_drawable_element_still_renders_inside_a_breakpoint():
+    files = generate({"type": "text", "text": "hi", "x": 0, "y": 0,
+                      "minWidth": 64, "color": "#ffffff"})
+    src = files["manager.py"]
+    assert "if width >= 64:" in src
+    assert "draw_text" in src
+
+
+def test_renderable_types_match_the_template_branches():
+    """The constant and the template must agree.
+
+    A type listed in the constant with no branch emits an empty block (the bug
+    above); a type with a branch but missing from the constant is silently
+    dropped from every generated plugin. Neither is visible without this check.
+    """
+    branches = set(re.findall(r"el\.type == '([a-z_]+)'", TEMPLATE.read_text()))
+    assert branches == set(C._RENDERABLE_ELEMENT_TYPES)
+
+
+def test_template_closes_the_branch_chain_with_a_fallback():
+    """Belt and braces: even if the constant drifts, no empty block escapes."""
+    text = TEMPLATE.read_text()
+    assert "{% else %}" in text
+    assert "pass  # element type" in text