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>

docs/PLUGIN_ARCHITECTURE_SPEC.md

@@ -190,7 +190,8 @@ class BasePlugin(ABC):
         """
         Fetch/update data for this plugin.
         Called every get_update_interval() seconds when that returns a
-        number, otherwise every manifest update_interval seconds.
+        number, otherwise at the static interval: the manifest's
+        update_interval, else the plugin config's update_interval, else 60s.
         """
         pass