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

web_interface/templates/v3/composer/manager.py.j2

@@ -288,6 +288,8 @@ class {{ class_name }}(BasePlugin):
 {{ pi }}        [_pb_x, _pb_y, _pb_x + _pb_fill_w, _pb_y + {{ el.bar_height }}],
 {{ pi }}        fill={{ el.fill_tuple }},
 {{ pi }}    )
+{% else %}
+{{ pi }}pass  # element type "{{ el.type }}" draws nothing
 {% endif %}
 {% endfor %}
             # ── End elements ───────────────────────────────────────────────