feat: add clipboard button component

docs/CHECKLIST.md

@@ -11,6 +11,26 @@ is the first downstream consumer; its screens drive the order.
 
 ## Queue — to follow
 
+Gaps surfaced by the signed comparison; ordered by signed's usage.
+
+- [ ] **Virtual list** — generic virtualized variable-height rows with a
+      scroll handle, promoted from the editor views' pattern over gpui's
+      `list`; signed's PR lists and commit diff drive it.
+- [ ] **Editable code field** — syntax-highlighted editable multiline field,
+      the code view's highlighter over the input's editing model; signed's
+      repo file browser.
+- [ ] **Separator** — standalone horizontal/vertical rule; menu-item
+      separators stay menu-scoped.
+- [ ] **Resizable group** — stacked arbitrary panes with drag dividers
+      outside the dock; signed's commit-diff split.
+- [ ] **Searchable list filter** — searchable collection behind combobox and
+      list option sources; the combobox's pluggable filter covers part of it
+      (signed's `SearchableVec`).
+- [ ] **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.
 - [ ] **Markdown view follow-ups** — streaming `append` with a coalesced
       background parse, text selection with copy, per-block a11y roles,
       GFM alerts + table column alignment, per-fence highlight reuse,
@@ -59,6 +79,8 @@ is the first downstream consumer; its screens drive the order.
 - [x] Command palette — filtered actions; the matcher stays caller-owned
 - [x] Scrollbar — overlay thumbs for gpui's `list` and `uniform_list`,
       platform autohide with a `visibility` override
+- [x] Clipboard button — copies a value, confirms with a brief check mark
+      that reverts itself
 
 ### data (`crates/data`)