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>

web_interface/app.py

@@ -157,6 +157,7 @@ _health_monitor_initialized = False
 # Register blueprints
 from web_interface.blueprints.pages_v3 import pages_v3
 from web_interface.blueprints.api_v3 import api_v3
+from web_interface.blueprints.composer import composer_bp
 
 # Initialize managers in blueprints
 pages_v3.config_manager = config_manager
@@ -173,6 +174,11 @@ api_v3.operation_queue = operation_queue
 api_v3.plugin_state_manager = plugin_state_manager
 api_v3.operation_history = operation_history
 api_v3.health_monitor = health_monitor
+
+composer_bp.config_manager = config_manager
+composer_bp.plugin_manager = plugin_manager
+composer_bp.plugins_dir = plugins_dir
+composer_bp.project_root = project_root
 # Initialize cache manager for API endpoints
 from src.cache_manager import CacheManager
 api_v3.cache_manager = CacheManager()
@@ -200,6 +206,7 @@ except Exception as _hm_err:  # pragma: no cover - defensive startup guard
 app.register_blueprint(pages_v3, url_prefix='')
 app.register_blueprint(pages_v3, url_prefix='/v3', name='pages_v3_legacy')
 app.register_blueprint(api_v3, url_prefix='/api/v3')
+app.register_blueprint(composer_bp, url_prefix='/composer')
 
 # Route to serve plugin asset files (registered on main app, not blueprint, for /assets/... path)
 @app.route('/assets/plugins/<plugin_id>/uploads/<path:filename>', methods=['GET'])