fix: keep the active search filter after deleting a track

[MiMoHo]

Type of change(s)

• Bug fix

What changed and why

When you search on the Tracks tab and then delete one of the matching tracks, the visible list would reset to the entire library instead of staying filtered.

Deleting a selected track posts Events.RefreshFragments(). MainActivity.shouldRefreshFragments() then calls refreshAllFragments(), which re-runs TracksFragment.setupFragment(). That method reloads the full library on a background thread and, back on the UI thread, calls updateItems(tracks) with the complete, unfiltered list and an empty highlight string. The search bar itself stays open (the query is still held by MySearchMenu), but the fragment never re-applies it, so the user is shown the whole library again.

The fix tracks the active query in a new lastSearchQuery field (set in onSearchQueryChanged, cleared in onSearchClosed) and, after the async reload finishes and the adapter is updated, re-applies the filter by calling onSearchQueryChanged(lastSearchQuery) when a query is still active. The filtered view (minus the deleted track) is now preserved.

Tests performed

• detekt, lint, unit tests and the build all pass locally (CI-equivalent).
• Source-verified: the tracks list now remembers the active search query and re-applies it after a refresh (e.g. following a delete), so search results are no longer replaced by the full library.
• Note: verified via CI + code review; the search-then-delete UI flow was not scripted end-to-end.

Closes the following issue(s)

• Closes Search feature not working correctly if you delete a file that you served for #85

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (verified via CI + source review; see Tests performed).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.

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