Adds a prompt builder that serialises 7-day mood/activity data, wires
InsightNotifier to call the on-device LLM directly (no user interaction),
caches results by prompt hash, and renders the analysis as a passive card.
Includes loading, empty (<2 entries), and error states with a force-refresh button.
Introduces AppShell with an IndexedStack-backed NavigationBar (Today,
History, Insights, Settings). Stub screens added for Calendar, Insights,
and Settings; app.dart now routes to AppShell instead of LogMoodScreen directly.
stefwill