fix: show matched memory count during search

[amandeavor]

Summary

• show the search result total while a query is active
• restore the store-wide total when the query is cleared
• cover matched, zero-result, and cleared-search states with regression tests

Closes #288

Validation

• bun test tests/memory-count.test.ts (2 passed)
• bun run typecheck
• production web build
• Prettier check on changed files
• git diff --check

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[lindixu6-hash]

Reviewed the search/count state flow and the extracted helper. The active-search path uses the API result total, clearing search restores the store-wide stats total, and zero-result search remains distinguishable from the unfiltered state. Local verification: targeted test 2/2, full suite 449/449 with 1 skip, typecheck, and production build all pass at 9fca346.