feat: Add start-to-current weight delta in stats display

[tryonlinux]

Display weight change from first entry to latest in the stats header. Shows "Lost X lbs/kg", "Gained X lbs/kg", or "No change" based on delta. Includes comprehensive test coverage for all scenarios.

🤖 Generated with Claude Code

[github-actions]

✅ Test Results: PASSED

Test Output

=== RUN   TestCalculateBMI_MetricUnits
--- PASS: TestCalculateBMI_MetricUnits (0.00s)
=== RUN   TestCalculateBMI_ImperialUnits
--- PASS: TestCalculateBMI_ImperialUnits (0.00s)
=== RUN   TestCalculateBMI_MixedUnits_KgInches
--- PASS: TestCalculateBMI_MixedUnits_KgInches (0.00s)
=== RUN   TestCalculateBMI_MixedUnits_LbsCm
--- PASS: TestCalculateBMI_MixedUnits_LbsCm (0.00s)
=== RUN   TestCalculateBMI_Underweight
--- PASS: TestCalculateBMI_Underweight (0.00s)
=== RUN   TestCalculateBMI_Overweight
--- PASS: TestCalculateBMI_Overweight (0.00s)
=== RUN   TestFormatWeight
--- PASS: TestFormatWeight (0.00s)
=== RUN   TestFormatBMI
--- PASS: TestFormatBMI (0.00s)
=== RUN   TestFormatDate
--- PASS: TestFormatDate (0.00s)
=== RUN   TestRenderWeightsTableDelta
=== RUN   TestRenderWeightsTableDelta/weight_loss_shows_'Lost'
=== RUN   TestRenderWeightsTableDelta/weight_gain_shows_'Gained'
=== RUN   TestRenderWeightsTableDelta/no_change_shows_'No_change'
=== RUN   TestRenderWeightsTableDelta/metric_units_show_correctly
--- PASS: TestRenderWeightsTableDelta (0.00s)
    --- PASS: TestRenderWeightsTableDelta/weight_loss_shows_'Lost' (0.00s)
    --- PASS: TestRenderWeightsTableDelta/weight_gain_shows_'Gained' (0.00s)
    --- PASS: TestRenderWeightsTableDelta/no_change_shows_'No_change' (0.00s)
    --- PASS: TestRenderWeightsTableDelta/metric_units_show_correctly (0.00s)
=== RUN   TestGoalDifferenceCalculation
=== RUN   TestGoalDifferenceCalculation/Need_to_lose_weight
=== RUN   TestGoalDifferenceCalculation/Need_to_gain_weight
=== RUN   TestGoalDifferenceCalculation/At_goal_weight
=== RUN   TestGoalDifferenceCalculation/Small_amount_to_lose
=== RUN   TestGoalDifferenceCalculation/Large_amount_to_gain
--- PASS: TestGoalDifferenceCalculation (0.00s)
    --- PASS: TestGoalDifferenceCalculation/Need_to_lose_weight (0.00s)
    --- PASS: TestGoalDifferenceCalculation/Need_to_gain_weight (0.00s)
    --- PASS: TestGoalDifferenceCalculation/At_goal_weight (0.00s)
    --- PASS: TestGoalDifferenceCalculation/Small_amount_to_lose (0.00s)
    --- PASS: TestGoalDifferenceCalculation/Large_amount_to_gain (0.00s)
=== RUN   TestGoalWeightWithWeightEntries
--- PASS: TestGoalWeightWithWeightEntries (0.03s)
=== RUN   TestGoalWeightEdgeCases
=== RUN   TestGoalWeightEdgeCases/Normal_goal
=== RUN   TestGoalWeightEdgeCases/Zero_goal
=== RUN   TestGoalWeightEdgeCases/Large_goal
=== RUN   TestGoalWeightEdgeCases/Small_goal
=== RUN   TestGoalWeightEdgeCases/Decimal_goal
--- PASS: TestGoalWeightEdgeCases (0.04s)
    --- PASS: TestGoalWeightEdgeCases/Normal_goal (0.01s)
    --- PASS: TestGoalWeightEdgeCases/Zero_goal (0.01s)
    --- PASS: TestGoalWeightEdgeCases/Large_goal (0.01s)
    --- PASS: TestGoalWeightEdgeCases/Small_goal (0.01s)
    --- PASS: TestGoalWeightEdgeCases/Decimal_goal (0.01s)
=== RUN   TestAddAndGetWeights
--- PASS: TestAddAndGetWeights (0.01s)
=== RUN   TestDeleteWeight
--- PASS: TestDeleteWeight (0.01s)
=== RUN   TestModifyWeight
--- PASS: TestModifyWeight (0.01s)
=== RUN   TestGetWeightsBetweenDates
--- PASS: TestGetWeightsBetweenDates (0.01s)
=== RUN   TestGoalWeightSetting
--- PASS: TestGoalWeightSetting (0.01s)
=== RUN   TestUpdateGoalWeight
--- PASS: TestUpdateGoalWeight (0.01s)
=== RUN   TestGoalWeightDifferentUnits
=== RUN   TestGoalWeightDifferentUnits/Imperial
=== RUN   TestGoalWeightDifferentUnits/Metric
=== RUN   TestGoalWeightDifferentUnits/Mixed_LbsCm
=== RUN   TestGoalWeightDifferentUnits/Mixed_KgIn
--- PASS: TestGoalWeightDifferentUnits (0.03s)
    --- PASS: TestGoalWeightDifferentUnits/Imperial (0.01s)
    --- PASS: TestGoalWeightDifferentUnits/Metric (0.01s)
    --- PASS: TestGoalWeightDifferentUnits/Mixed_LbsCm (0.01s)
    --- PASS: TestGoalWeightDifferentUnits/Mixed_KgIn (0.01s)
PASS
ok  	github.com/tryonlinux/thicc/tests	0.164s