Migrates FilterEdit.tags_buf and lang_buf from String to InputBuffer
(the same display-width-aware type used by Search/Ask), adds cursor
placement in render_filter_overlay so the caret sits at the end of the
focused field, and adds a Hangul filter test
(filter_overlay_accepts_hangul_tags) that confirms wide chars round-trip
through key events and commit_into unchanged.
Also adds App::library_filter_for_testing accessor mirroring
populate_library_for_testing style.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
altair823
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