[feat-023] Filter UX: dedupe single-kind filters + anchor popover right

Two real bugs from smoke testing:

- Adding the same flag/verified filter multiple times stacked them and
  silently AND'd the conditions (Flag: Yellow + Flag: Yellow,Red →
  matches docs that have BOTH a yellow cell AND a {yellow|red} cell).
  Worse, Verified-only AND Unverified-only is logically empty (no cell
  is both).
  Fix: at most one Flag filter and one Verified filter. Selecting
  either kind in the Add Filter popover replaces the existing one
  rather than stacking. Text predicates can still stack (different
  columns are useful AND-combinations).
  The popover form pre-fills from the existing filter of the same
  kind, so re-opening edits in place.

- The popover anchored at left-0 of the trigger; with many active
  filters the trigger sits at the right end of the row and the
  popover ran off the right edge of the viewport, half-clipped.
  Fix: anchor at right-0 so it grows leftward from the trigger and
  always stays in-viewport.

Bonus: the popover's default tab now picks the *missing* kind
(Flag → Verified → Column value), so building a multi-facet filter
set takes one fewer click.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>