Fix: slider logs with default value can't be saved

[mapgie]

Summary

• A slider tracker entry shows numericMin as its default value, but LogCategoryViewModel.numericValue stays null until the user drags the slider.
• save() treated numericValue == null as "nothing to save" and returned early, so tapping Save without touching the slider silently did nothing.
• Now falls back to category.numericMin (the value already shown to the user) when numericValue is null.

Changes

• LogCategoryViewModel.kt: fix the early-return in save() for numeric_slider
• Version bump to 0.47.2-beta.1 (versionCode 102) + changelog entry
• LESSONS.md: added a transferable lesson about null "not yet interacted" state vs. save validation

Test plan

• Open a slider tracker item, don't touch the slider, tap Save - entry should be saved with the default (min) value
• Move the slider and save - existing behaviour unchanged
• Edit an existing slider log - existing behaviour unchanged

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