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>
