v0.6.2

Grade calculation improvements

• Loads detail data from GET /finalgrades/{id}.
• Uses student-visible beste.schule calculation_rule formulas when available.
• Respects every grade collection weighting.
• Uses a weighted 50/50 SO/KA fallback when no API rule is visible.
• Uses the weighted category average for subjects with only one grade category.
• Shows formula source, rule, variables and result in grade sensor attributes.
• Removes the empty api_average attribute and the previous double-classwork fallback.