fix: analytics charts — year separators, partial month, dead code, i18n

[dholbach]

Summary

• Chart correctness: Exclude current partial month from all trend charts (capacity, revenue, expenses) so the last data point is never anomalously low; fix year separator/label misalignment in all 4 line charts that use findFirstNonZeroIndex (year labels were shifted by however many leading months were trimmed)
• Refactor: Extract _build_holiday_set() in capacity_helpers.py; replace manual month-end arithmetic with DateRangeHelper.get_last_of_month; get_capacity_trends loop delegates to _calculate_weighted_capacity instead of duplicating the formula
• Dead JS removed: setupTooltip, setupBarChart, drawXAxisLabels, calculateYearLabels; drawSimpleBarChart inline label replaced with drawYearLabel() call; chart_tooltip.js script tag removed
• i18n (P-039): CHART_MESSAGES JS global for canvas empty-state strings; LOCALE from navigator.language instead of hardcoded 'de-DE'; MONTH_LABELS and 'Ø' legend wrapped with {% trans %}; practice_analysis.py insight strings and "Arbeitstage" wrapped with gettext/ngettext; German translations added to django.po

Details

Partial month guard

Both get_capacity_trends (capacity_helpers.py) and _get_monthly_aggregation (analytics_utils.py) now cap end_date at the last day of the previous complete month when called with a date in the current month. A month with only 2 working days has 8 h capacity — including it makes the last chart point look like a sudden crash.

Year separator alignment

findFirstNonZeroIndex slices leading zero-months before rendering, but drawYearSeparators/drawYearLabels assumed index 0 = January of startYear. If data starts in, e.g., March 2020, the 2021 separator appeared in July 2021 instead of January 2021. All four affected charts (revenueLineChart, daysToPaymentChart, cancellationTrendsChart, capacityTrendsChart) now parse startMonth from their first label and pass it through.

revenueLineChart also had {{ month.month|slice:":3" }} which truncated "01/20" to "01/", making parseMonthString return null. Now emits full "MM/YY" labels.

Test plan

• Analytics page renders without JS errors on all four tabs
• Revenue trend chart year labels (2021, 2022, …) align with January of each year
• Capacity trends chart ends at June 2026 (last complete month), not July
• Revenue trends chart ends at June 2026
• Year separators align correctly for all line charts
• Canvas empty-state messages appear in German
• Month abbreviations in the year comparison chart are German (Mär, Mai, Okt, Dez)
• "Avg" legend label renders as "Ø" in German locale
• ./dev.py test passes

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