Design column header filter popovers

[Zhortein]

Objective

Create a design proposal for column header filter popovers.

Context

Column filters currently render in the toolbar. The desired UX is to show a small filter icon in the column header and open a compact filter popover/dropdown when clicked.

This is more modern and saves toolbar space, but it impacts header rendering, filtering state, accessibility and Stimulus behavior.

Scope

• Document the proposed UX.
• Decide between Bootstrap dropdown and Bootstrap popover.
• Define markup.
• Define Stimulus behavior.
• Define accessibility requirements.
• Define request parameter behavior.
• Define how active filter state is shown in headers.
• Create follow-up implementation issues.

Out of scope

• Implementation.
• New JavaScript dependencies.
• Complex SearchBuilder expressions.

Constraints

• Bootstrap-first.
• Vanilla JavaScript only.
• No jQuery.
• Keep filters explicit and typed.
• Must remain accessible.

Acceptance criteria

• Proposal document exists.
• UX approach is chosen.
• Implementation issues are created.
• Current limitations are explicit.
• QA passes.