Show active category filters as chips on the statistics page

[luflow]

Summary

• The category filter on the statistics page was easy to miss once set — the only sign it was active was the trigger button's own color, no different from the other filter dropdowns. Selected categories now render as closable chips below the filter row, mirroring the "All appointments" list's active-filters row.
• Added a small "Filter" label above the category button (matching "Period"/"Range" next to it), and the category trigger now always renders as a proper (secondary-variant) button so it reads as a filter control at a glance.
• Moved the export button to the right edge of the filter row.
• Extracted the category-chip resolution and label-splitting logic — previously copy-pasted between the statistics page and the appointment list — into a shared useCategoryFilterChips composable, used by both views now.

Test plan

• npm run lint (eslint)
• npm run stylelint
• npm run build (vite)
• scripts/check-german-l10n.py / scripts/check-source-strings.py
• Manual check in browser (not run in this environment)

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