Add Metrics tab with list, detail, and time range selector

[Jds-23]

Summary

• Add Metrics as 6th tab (before Settings) with FlatList of metric cards showing name, latest value, unit, and trend arrow
• Add metric detail screen (/metric/[id]) with View-based bar chart, entry history list, and time range selector (1W/1M/3M/6M/1Y)
• Pure utility functions: computeTrend (up/down/flat) and getStartDateForRange

Closes #51

Test plan

• 10 unit tests for computeTrend and getStartDateForRange
• 4 tab screen tests: empty state, metric cards, trend arrows, navigation
• 4 detail screen tests: name + chart, dates + values, back button, range pills
• 253 total native tests pass (0 regressions)
• pnpm check — 0 lint errors
• Manual: pnpm dev:native → tap Metrics tab, see list, tap metric, see chart + range selector

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