fix(composer): register composer_bp and close a second empty-block gap

composer_bp defined the whole Plugin Composer feature -- the /composer/
page and all its API routes -- but web_interface/app.py never imported or
registered it, so every composer URL 404'd in the running app (every
composer test builds its own minimal Flask app and registers the
blueprint directly, which is why this went unnoticed). Wire it up the
same way pages_v3/api_v3 are: import, set config_manager/plugin_manager/
plugins_dir/project_root, register_blueprint(url_prefix='/composer') --
matching the prefix composer.html and composer-app.js already hardcode.

Also closes the other still-open half of a CodeRabbit finding: manager.py.j2
already guards element types the template has no branch for, but a
dynamic_text element with binding.source other than 'config' hit the same
empty-if-block bug one level deeper (its own inner if produced nothing).
Added the same pass fallback.

Verified both against current code before fixing; the other 8 findings
from that review were already fixed in earlier commits on this branch.

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

test/test_composer_empty_block.py

@@ -78,3 +78,24 @@ def test_template_closes_the_branch_chain_with_a_fallback():
     text = TEMPLATE.read_text()
     assert "{% else %}" in text
     assert "pass  # element type" in text
+
+
+@pytest.mark.parametrize("wrapper", [
+    {"minWidth": 64},
+    {"blink": True},
+    {"minWidth": 64, "blink": True},
+])
+def test_dynamic_text_with_non_config_binding_does_not_break_generation(wrapper):
+    """dynamic_text only renders a body for binding.source == 'config'.
+
+    _preprocess_elements accepts any string as binding.source (it just
+    defaults a missing one to 'config'), so a 'live' or 'sensor' source --
+    anything a client sends that isn't literally 'config' -- hit the same
+    empty-if-block bug as an undrawable element type, just one level deeper:
+    the branch is taken, but its own inner `if` produced nothing.
+    """
+    element = {"type": "dynamic_text", "x": 0, "y": 0, "color": "#ffffff",
+               "binding": {"source": "live", "key": "temperature"}, **wrapper}
+    files = generate(element)          # must not raise ComposerInputError
+    assert "manager.py" in files
+    assert 'binding_source "live" draws nothing' in files["manager.py"]