* feat: add top-level persistent filters for keyword research
Add a collapsible filter bar below the search bar so users can set
default filters (volume, CPC, difficulty, include/exclude terms) that
persist across searches and sessions via localStorage.
* simplify: remove top-level filter UI, keep only localStorage persistence
Revert the search bar to its original state. The existing in-table
filters already provide good UX — all we needed was persistence.
Auto-expand the filter panel when persisted filters are active on load.
* feat: show filtered keyword count as "Showing X of Y keywords"
When filters are active, display "Showing 14 of 150 keywords" instead
of just "14 keywords". When no filters are active, show "Showing X
keywords". Applied to both desktop and mobile views.
* style: fix prettier formatting
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