fix: address CodeRabbit review on #580

- Preview fallbacks (SSE stream and /display/current) use logical_size({})
  (128x32, the shared default) instead of a hard-coded 128x64.
- display_geometry treats a non-mapping display/hardware block as missing,
  so a malformed config.json falls back to defaults instead of raising
  AttributeError (which turned the Starlark render into an HTTP 500).
- Docs: the static update interval falls back manifest -> plugin config
  -> 60s, in both the API reference and the architecture spec.

Not taken: validating double_sided copies against chain_length/parallel.
An orientation Rotate: or U-mapper pixel mapper decides which axis panels
lie on, so the counts would reject working setups (the existing
vertical-split test is one).

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

test/test_display_geometry.py

@@ -71,6 +71,13 @@ def test_disabled_double_sided_is_ignored():
     assert logical_size(cfg) == (256, 32)
 
 
+@pytest.mark.parametrize('display', ['oops', ['a'], 1, {'hardware': 'oops'},
+                                     {'hardware': ['a']}])
+def test_non_mapping_display_config_uses_the_defaults(display):
+    assert physical_size({'display': display}) == (128, 32)
+    assert logical_size({'display': display}) == (128, 32)
+
+
 @pytest.fixture
 def display_client(monkeypatch):
     from web_interface.blueprints.api_v3 import api_v3
@@ -103,6 +110,15 @@ def test_display_current_defaults_chain_length_like_display_manager(display_clie
     assert (data['width'], data['height']) == (64 * DEFAULT_CHAIN_LENGTH, 32)
 
 
+def test_display_current_falls_back_to_the_shared_default(display_client):
+    client, config_manager = display_client
+    config_manager.load_config.side_effect = ValueError('unreadable')
+
+    data = client.get('/api/v3/display/current').get_json()['data']
+
+    assert (data['width'], data['height']) == logical_size({}) == (128, 32)
+
+
 def test_preview_callers_do_not_rederive_the_size():
     """Every preview/size caller goes through display_geometry, so none of
     them can drift back to a private chain_length default."""