Toolbar: drag commands from drop-downs onto the toolbar; pin icon; Customize Toolbar… (#295)

* Toolbar drop-downs highlight the row under the pointer and the keyboard focus (#288)

A panel drop-down's command rows are full-width rows that take the theme's hover background
(icon, label and shortcut on it) when hovered or focused. Up/Down move the focus through the
enabled rows, Enter starts the command, Escape closes the drop-down. Rows publish
`dropdown:<id>` and the highlighted one `dropdown_hot:<id>`; scenario s46 checks pointer and
keyboard highlight.

Fixes #288

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

* Toolbar: drag commands from drop-downs onto the toolbar; pin icon; Customize Toolbar… (#289)

- A panel drop-down row drags onto the toolbar (a button or a panel's free space, with the
  drop line) and is pinned there; the dragged command's icon follows the pointer.
- A toolbar button dragged off the toolbar, or back into the open drop-down, leaves it (its
  icon shows a cross while releasing would remove it).
- A hovered drop-down row shows a pin at its end: click to pin or unpin.
- Right-click on the toolbar's empty space: Customize Toolbar… (a window listing the tab's
  commands with check boxes, and Reset Toolbar) and Reset Toolbar; command menus have
  Customize Toolbar… too.
- Still per workspace and tab, kept with the preferences. Toolbar buttons also publish
  `button:<id>`; scenario s47 drags a row to the toolbar, unpins by dragging off and into the
  drop-down, uses the pin icon and Customize Toolbar…, and checks it survives a restart.

Fixes #289

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

* Toolbar drop-down pin: a push pin drawn in code (upright and filled when pinned); control protocol lists the toolbar handles

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

---------

Co-authored-by: Claude Opus 5.5 <[email protected]>

docs/control-protocol.md

@@ -63,7 +63,7 @@ unit regardless of the design, pass an expression with one: `"value": "50 mm"` o
 | `ui.sketchToScreen` | `points`: `[[x, y]…]` in active-sketch coordinates | their screen points (null when behind the camera) |
 | `ui.dialogInput` | `index` | what clicking an input's Select chip does: picks go to that input |
 | `ui.at` | `world: [x,y,z]` \| `sketch: [x,y]` \| `plane: "XY"` \| `axis: "Z"` \| `dimension: "d1"` (its value text) \| `handle: "arrow"` (a manipulator handle, see `scenario::publish_handle`) | the screen point (a scenario step's target; see `crates/ui-egui/src/scenario.rs`) |
-| `ui.handles` | `prefix?` | the handles drawn last frame, `[{name, x, y}]`: toolbar buttons and drop-down items `toolbar:<command id>`, panel labels `panel:<tab>:<panel>`, browser rows `row:<label>` and fold arrows `fold:<label>`, manipulator handles; orientation tips `axis_view:x`, `axis_view:-x` (and y/z) |
+| `ui.handles` | `prefix?` | the handles drawn last frame, `[{name, x, y}]`: toolbar buttons and drop-down items `toolbar:<command id>` (the button alone: `button:<command id>`; a drop-down row: `dropdown:<command id>`, highlighted `dropdown_hot:<command id>`, its pin `dropdown_pin:<command id>`; Customize Toolbar's check boxes `customize:<command id>`), panel labels `panel:<tab>:<panel>`, browser rows `row:<label>` and fold arrows `fold:<label>`, manipulator handles; orientation tips `axis_view:x`, `axis_view:-x` (and y/z) |
 | `ui.editFeature` | `feature` (id or name) | what double-clicking a timeline item does: rolls back to it and opens its dialog filled in (sketches open in sketch mode) |
 | `ui.scroll` | `x`, `y`, `delta?` | wheel zoom at the cursor |
 | `ui.key` | `key` (egui key name, `Enter`, `Escape`…), `cmd?`, `shift?` | |