feat: add SQL operator symbols to filter menu

[datlechin]

Summary

• Add symbol computed property to FilterOperator with SQL operator symbols (=, !=, LIKE %..%, >, >=, etc.)
• Filter operator picker now shows symbols alongside localized names (e.g. = equals, LIKE %..% contains)
• FilterDefaultOperator.displayName delegates to FilterOperator for consistent formatting
• Fix layout bug where no-value operators (IS NULL, IS EMPTY) caused the filter row to center

Closes #834

Test plan

• Open filter panel (Cmd+Shift+F), click operator dropdown — verify symbols appear with correct format
• Select "is NULL" / "is not NULL" — verify no symbol prefix, row stays left-aligned
• Open filter settings (⋯ > Filter Settings) — verify default operator picker shows consistent symbol format
• Switch to a non-English language — verify symbols remain while text is translated
• Enable VoiceOver, navigate to operator picker — verify it reads display names not symbols