feat(#537): analytics screen — usage + per-model stats (#558)

* feat(#537): analytics screen — usage + per-model stats from /api/analytics/*

* fix(#537): address PR review — cancel-in-flight loads, retain cache on refresh, drop redundant async

* fix(#537): surface real analytics failure, mirror backend shape (period_days/tools), round-trip test

* fix(#537): tolerate float last_used_at in analytics (backend emits epoch as number)

* fix(#537): type tools[] as AnalyticsToolUsage (tool/count/percentage), render Top tools section

* fix(analytics): fix crashes and refactor tab layout

* perf(analytics): cache responses and prefetch day windows

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Co-authored-by: M57 <m57@hermes-control.app>

app/src/main/java/com/m57/hermescontrol/NavigationKeys.kt

@@ -50,3 +50,5 @@ import kotlinx.serialization.Serializable
 @Serializable data object KanbanScreen : NavKey
 
 @Serializable data object ProcessesScreen : NavKey
+
+@Serializable data object AnalyticsScreen : NavKey