Capture growth weight as pounds and ounces

[mike12806]

What

The Add/Edit Measurement dialog only accepted a single decimal weight, which doesn't match how baby weights are actually reported ("7 lb 4 oz"). When the weight unit is lb, the form now shows a companion oz field.

How

• New client/src/utils/weight.ts with lbOzToPounds, poundsToLbOz, and formatWeight.
• The lb + oz pair is combined into the decimal pounds value that is already persisted, so no API or schema changes — weight stays a single number with a weight_unit.
• Editing an existing pound entry splits the stored decimal back into lb + oz (ounces keep one decimal place so half-ounce scales round-trip).
• Pound weights display as "7 lb 4 oz" on the history cards and the weight stat tile; other units keep their existing <value> <unit> rendering and single-field entry.
• Switching the unit away from lb hides and clears the oz field.

Testing

• client/test/weight.test.ts — conversion and formatting, including round-trips and the 16 oz carry.
• client/test/GrowthWeight.test.tsx — dialog behavior: lb + oz saves 7.25, oz-only saves 0.75, blank saves null, oz field hidden for metric units, and an existing 7.25 lb entry renders as "7 lb 4 oz" and round-trips through edit.
• Full client suite (111 tests) and tsc --noEmit pass. Layout verified at 390px width — the Unit dropdown stays aligned with the Height and Head Circumference rows.

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