Adds a month grid heatmap (colour-coded by daily mood average), prev/next
month navigation, and a scrollable entry list below for the selected day
showing mood, time, note, and metric chips (sleep, energy, positivity,
self-worth).
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