Adds src/components/reports/FilterPanel.tsx, the shared filter bar for the
report pages. It renders the page's own temporal control (PeriodSelector /
CompareReferenceMonthPicker / YearNavigator / …) as-is via a `temporalControl`
ReactNode prop, plus a checkbox multi-select over `accounts: ImportSource[]`.
No `temporalMode` enum — that design was rejected to avoid mixing state
sources; the panel owns no temporal state at all.
Empty `accountIds` means no filter (all sources). Selection toggling is a
pure, exported `toggleAccountId` helper. Copy says "sources"/"import
sources" everywhere, never "comptes"/"accounts", so the filter reads as
distinct from the Bilan module's own "Compte" vocabulary.
This issue creates the standalone component only; wiring it into report
pages is left to follow-up issues (#275/#276).
Resolves#274
