fix(dashboard): bound sparkline extrema scan - #7426

Summary

• replace Math.max(...data) and Math.min(...data) with a bounded single-pass extrema scan
• prevent large sparkline data sets from exceeding the JavaScript function argument limit
• add a regression that renders 150,000 points, above the local runtime spread limit

Inventory finding: F-a7b4a9b4917dc6d0

Verification

• corepack pnpm@10.33.0 --dir crates/librefang-api/dashboard exec vitest run src/components/ui/Sparkline.test.tsx (2 passed)
• corepack pnpm@10.33.0 --dir crates/librefang-api/dashboard test -- src/components/ui/Sparkline.test.tsx (101 files, 1076 tests passed)
• corepack pnpm@10.33.0 --dir crates/librefang-api/dashboard typecheck
• corepack pnpm@10.33.0 --dir crates/librefang-api/dashboard lint
• corepack pnpm@10.33.0 --dir crates/librefang-api/dashboard build
• git diff --check
• pre-commit and pre-push hooks

Out of scope

• chart downsampling or virtualized rendering for very large series
• changing handling of non-finite input values
• other dashboard chart components