Extract shared weekday aggregation helpers and unify charts

[IanTheMitten]

Motivation

• Remove duplicated weekday/date aggregation logic in budget and statistic codepaths to prevent divergence and make maintenance easier.
• Provide a single source of truth for weekday revenue aggregation so both chart components and sampling utilities share identical behavior.

Description

• Added src/components/analytics/sharedAggregation.ts with WEEKDAY_LABELS, getDayKey, parseDayKey, isWeekday, getWeekKey, getWeekdayRevenueByDate, and buildWeekdayAverageRevenueChartData to centralize weekday/date helpers and chart bucketing logic.
• Updated src/components/budget/WeekdayRevenueBarChart.tsx and src/components/statistic/WeekdayRevenueBarChart.tsx to consume getWeekdayRevenueByDate and buildWeekdayAverageRevenueChartData from the shared module and added an import-comment near the imports to document the shared source for future maintainers.
• Refactored src/components/budget/analyticsSampling.ts and src/components/statistic/analyticsSampling.ts to use the shared getDayKey, parseDayKey, getWeekKey, and getWeekdayRevenueByDate helpers instead of local duplicate implementations, preserving existing sampling APIs.
• Kept chart presentation and public sampling APIs unchanged; each chart now calls the shared builder with page-specific sampled date arrays.

Testing

• Ran the production build with npm run build, which completed successfully (Vite build completed without errors).

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