fix: move long numbers to the next unit when they round up - #4658

What

formatLongNumber (src/lib/format.ts) picks the unit from the raw value and then rounds with toFixed, so values just below a unit boundary round up to an impossible label:

• 999999 → 1000k (should be 1.0m)
• 999999999 → 1000.0m (should be 1.0b)
• 99999 → 100.0k (should be 100k)
• 9999 → 10.00k (should be 10.0k)
• 999.6 → 1000 (should be 1.00k)

formatLongNumber is used in the link and pixel metrics bars and in the attribution, funnel, goal, heatmap and journey reports, so the wrong labels show up in all of them.

Fix

Lower each threshold by half of the smaller format's last digit (999950000, 999500, 99950, 9995, 999.5). A value that would round up to the next boundary now uses the next format instead. Values outside these small ranges format exactly as before.

Tests

Added a formatLongNumber test in src/lib/format.test.ts that covers the value just above and just below each new threshold.

• Without the fix, the new test fails with expected '1000.0m' to be '1.0b'.
• With the fix, npx vitest run src/lib/format.test.ts passes all 9 tests.
• biome lint is clean on both changed files.

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