fix(composer): close binding_source code injection, line-align, and project_root UnboundLocalError

Three findings from CodeRabbit's review of 6c23994b, all verified against
current code before fixing:

- manager.py.j2 interpolated binding.source unescaped into a Python comment
  (`pass  # dynamic_text binding_source "{{ el.binding_source }}" draws
  nothing`). A source string with a newline broke out of the comment; a
  crafted payload produces a clean, ast.parse-valid `import os` in the
  generated plugin (confirmed against the pre-fix template). This is now a
  fixed literal comment that never interpolates the value. Live now that
  composer_bp is registered. CWE-94.
- _alignElement moved a line's x0 (or y0) to the new position but left x1
  (or y1) behind, so aligning a line changed its shape instead of moving
  it. Both endpoints now translate by the same delta.
- web_interface/app.py only assigned project_root inside the relative-path
  branch of the plugins_dir resolution. An absolute plugin_system.plugins_
  directory (a supported config value) hit UnboundLocalError importing the
  module at all, since SchemaManager/composer_bp use project_root further
  down. Now assigned unconditionally before the branch.

Also extends BOUND_TYPES coverage in composer-app.js (_isBound,
removeConfigVar, _validateBeforeExport) from dynamic_text/progress_bar to
all six element types that carry a binding object (countdown, pips,
sparkline, gauge too) -- found by direct code reading against
ELEMENT_DEFAULTS in composer-canvas.js, not from a review comment. Without
it, those four types could export with an unbound config key with no
validation error, and deleting a config var they used gave no warning.

All four fixes have mutation-checked regression tests (fail against the
reverted code, pass with the fix): test_binding_source_cannot_break_out_of_the_comment_it_lands_in,
test_align_translates_both_line_endpoints_not_just_the_start,
test_app_plugins_dir_resolution.py, test_binding_checks_cover_every_bound_element_type.

Full suite: 4365 passed, 58 skipped, 2 failed -- both the pre-existing
Europe/Kiev/Asia/Calcutta tzdata-alias gap on this sandbox, identical on
origin/main, unrelated to this change.

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

web_interface/app.py

@@ -83,11 +83,11 @@ plugin_system_config = config.get('plugin_system', {})
 plugins_dir_name = plugin_system_config.get('plugins_directory', 'plugin-repos')
 
 # Resolve plugin directory - handle both absolute and relative paths
+project_root = Path(__file__).parent.parent
 if os.path.isabs(plugins_dir_name):
     plugins_dir = Path(plugins_dir_name)
 else:
     # If relative, resolve relative to the project root (LEDMatrix directory)
-    project_root = Path(__file__).parent.parent
     plugins_dir = project_root / plugins_dir_name
 
 plugin_manager = PluginManager(