feat: add dropdown menu component

docs/CHECKLIST.md

@@ -22,8 +22,6 @@ Gaps surfaced by the signed comparison; ordered by signed's usage.
 - [ ] **Editable code field** — syntax-highlighted editable multiline field,
       the code view's highlighter over the input's editing model; signed's
       repo file browser.
-- [ ] **Dropdown menu** — trigger button + popover + menu composite, three
-      signed call sites; must stay a real component, not a wrapper.
 - [ ] **Motion easing helpers** — cubic ease-out and lerp beside
       `theme::Motion`; low priority, gpui's animation primitives cover most
       cases.
@@ -96,7 +94,13 @@ Gaps surfaced by the signed comparison; ordered by signed's usage.
       (and dark mode); the panel padding and separator margins come from
       `Theme::spacing`, and every action row renders at the stated
       `menu_item_height` with content centered, so the stated metric is the
-      whole row; rows carry debug selectors for headless geometry tests
+      whole row; rows sit a half-step apart (`gap_0p5`) so the rounded
+      highlight reads as its own chip, and the unselected pointer wash is the
+      preset's `muted_hover`, not a foreground token; pointer opens arm no
+      row — a keyboard open (arrow or Enter on the trigger) arms the first
+      selectable row, and Select arms its current value — so Enter cannot
+      fire an option nobody chose; rows carry debug selectors for headless
+      geometry tests
 - [x] Popover platform chrome — the panel border behind
       `Theme::borders.overlays` (and dark mode); the anchor gap is the
       spacing unit and rides `anchored().offset`, not a margin, so the
@@ -139,6 +143,12 @@ Gaps surfaced by the signed comparison; ordered by signed's usage.
       `Theme::borders.overlays` (and dark mode), taking the surface color
       where the platform paints none; it rides the dialog elevation, so dark
       mode's no-shadow rule applies to it too
+- [x] Dropdown menu — trigger button + menu composite reusing the menu's
+      anchored panel; the trigger takes the full button variant vocabulary;
+      the split form keeps the main zone a working button (`on_click` fires
+      anywhere outside the chevron) while the chevron zone opens the menu,
+      the menu-only form opens on activation, ArrowDown opens from focus, and
+      focus returns to the trigger on every close
 - [x] Switch platform chrome — the track and knob take the stated
       `switch_height` / `switch_width` / `switch_knob` (the knob's inset
       derived to center it in the track, its travel from the remaining