Align overview invoice focus filters

[mihape]

Summary

• add a visible invoice due-date filter for overdue and 7-day due invoices
• make Overview / Mai fókusz invoice cards open the matching filtered invoice list
• align Overview invoice date parsing with local YYYY-MM-DD handling
• exclude cash/KP invoices from open overdue focus totals even when older/imported data still says payment pending

Verification

• node --check for all JavaScript files covered by the Windows build workflow
• git diff --check
• targeted smoke check for open overdue counting excluding cash and paid invoices

Refs #43

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve missing due dates as non-overdue

When an imported or older record has dueDate: null, this fallback now calls new Date(null), which is a valid Jan 1, 1970 date; isPastDate(null) therefore returns true and undated open invoices or utility settlements are shown as overdue dashboard/focus items. The previous falsy guard skipped these records, so keep a falsy check here or parse the normalized string instead of the original value.

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