fix(schema): parse node-status banner and layout UI sections

The UI-schema loader walked a hardcoded list of flat TOML sections and
left [banners.*] and [layout] out, so those sections in ui.toml were
silently dropped. The node-status error strip at the top of the content
column could not be styled from the schema at all.

- The flat-section list is now one shared constant used by the file,
  string and overlay loaders, and it includes banners and layout.
- The node-status banner is themeable from ui.toml like every other
  section; a note in the toml records that these parse as flat sections.

No visible change unless ui.toml overrides those sections.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

res/themes/ui.toml

@@ -700,6 +700,7 @@ status-pill-bg-alpha = { size = 30 }
 status-pill-y-offset = { size = 1 }
 confirmed-threshold = { size = 10 }
 
+# NOTE: [banners.*] and [layout] are parsed as flat sections (kFlatSections in ui_schema.cpp).
 # Persistent node/RPC error strip at the top of the content column (see App::renderNodeStatusBanner).
 # Slightly taller than the per-tab sync banner so it comfortably holds the Reconnect/Restart action.
 [banners.node-status]