refactor(ui): extract shared DropdownMenu component

[forketyfork]

Issue

The diff view's "Send to agent" dropdown (diff_overlay.zig) and the selection dialog's agent picker (selection_agent_overlay.zig) were two independently implemented dropdowns. They shared AgentKind and rendering primitives, but each had its own hand-rolled hit-testing, hover tracking, and box/highlight rendering, which could drift out of sync over time.

Solution

Added src/ui/components/dropdown_menu.zig, a reusable DropdownMenu component that:

• Owns open/hover/keyboard-nav state and the committed selected index.
• Caches its own item-label textures (invalidated on font-cache generation/size/content changes).
• Reports a .selected / .closed event on click, Enter, or Escape, so the owning component reacts (persist the pick, or act on it immediately) instead of re-implementing hit-testing and highlight rendering.
• Takes menu items as a plain []const []const u8, so it's fully generic — not tied to AgentKind.

Both overlays were rewired onto it, removing their duplicated fields (show_agent_dropdown/agent_dropdown_hovered/selected_agent/agent_tex, and the local dropdownItemAt helper) in favor of a single agent_dropdown: DropdownMenu field each.

Two intentional behavior changes fell out of the unification:

• The diff view's dropdown now supports Up/Down/Enter keyboard navigation, which it didn't have before (only Escape worked).
• Both dropdowns now seed a highlighted row on open and capture all keyboard input while open, treating an open dropdown as modal to the keyboard — consistent with the selection dialog's prior behavior.

docs/ARCHITECTURE.md was updated with a new table row documenting the shared component and its consumers.

Context

None.

Test plan

• Open the diff view, add a review comment, click "Send to agent" — confirm the dropdown lists "Paste directly" + each agent, hover/click selection works, and picking an item sends as before.
• With the "Send to agent" dropdown open, try Up/Down/Enter and Escape — confirm keyboard navigation selects/dismisses correctly.
• Open the selection-agent dialog (select terminal text, launch agent flow), open the agent selector dropdown, and confirm click/hover/keyboard selection still works and the closed selector shows the chosen agent.
• Visually compare both dropdowns against main to confirm no unintended styling regressions (colors, radius, fade animation on the diff view's dropdown).

[Copilot]

Pull request overview

Extracts duplicated agent-picker behavior into a reusable dropdown component.

Changes:

• Adds shared dropdown state, rendering, caching, and tests.
• Migrates both agent dropdown consumers.
• Documents and registers the new component’s tests.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/ui/components/dropdown_menu.zig	Implements the shared dropdown.
src/ui/components/selection_agent_overlay.zig	Adopts the shared dropdown.
src/ui/components/diff_overlay.zig	Adopts dropdown keyboard and selection handling.
src/main.zig	Registers dropdown tests.
docs/ARCHITECTURE.md	Documents the component.

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