test: reorganize tests and add missing coverage
Extract app.rs tests to app_tests.rs following the driver test pattern.
Reduces app.rs from ~1900 to ~1080 lines (43% reduction).
Add ui/table_tests.rs with unit tests for pure functions:
- compute_col_widths (basic, empty, max cap, partial rows)
- visible_column_range (all fit, partial, scroll, edge cases)
- table_title (all visible, partial, scroll)
Add driver/mod.rs tests for extracted helpers:
- sqlite_value_to_string (all 5 Value variants + invalid UTF8)
- collect_active_filters (empty, single, multiple, all none)
Net: +226 lines of focused test coverage, better file organization.