refactor: DRY flowview filters page layout wrapper

[somethingwithproof]

Problem

flowview_filters.php repeats page wrapper layout handling for action rendering (top_header/bottom_footer) across action branches.

Proposed change

• add a shared layout wrapper helper for flowview filters page rendering
• route edit/default rendering through the shared helper
• preserve behavior including embed handling

Tests

• add standalone regression tests for helper call ordering and embed behavior
• add source assertions that action paths use the shared helper
• run syntax checks for touched files