Add a draggable sidebar resize handle

User reports the sidebar can't be resized. Until now sidebar width
was only mutable through the COLLAPSED/DEFAULT toggle and through
typed settings. Real browsers (Arc, Dia, Zen) all let you drag the
right edge of the sidebar to size it live.

Wire it through the existing space.sidebar_width_px:

* `ElyShell.sidebar_resize_origin: Option<(f32, u16)>` records the
  cursor-x and width-px at the moment the handle is grabbed.
* `begin_sidebar_resize` / `end_sidebar_resize` set and clear it.
* Window-level `on_mouse_move` consults the origin first; while held,
  it forwards the delta to `set_active_sidebar_width` clamped to
  220–480 px so you can't accidentally annihilate either pane.
* Window-level `on_mouse_up` releases the drag.
* The handle itself is a 6 px transparent strip pinned to the right
  edge of the expanded sidebar, `cursor_col_resize`, with a soft white
  hover. mouse-down captures the origin; the rest is window events.

cargo test --workspace: 440 passed, 0 failed.

crates/ely_app/src/shell/mod.rs

@@ -68,6 +68,10 @@ pub struct ElyShell {
     pub(crate) workspace_picker_open: bool,
     pub(crate) sidebar_hover_expanded: bool,
     pub(crate) command_selected_index: usize,
+    /// Live state for sidebar-resize drag. While the user holds the
+    /// resize handle: `(mouse_x_at_drag_start, sidebar_width_px_at_drag_start)`.
+    /// `None` whenever no drag is in flight.
+    pub(crate) sidebar_resize_origin: Option<(f32, u16)>,
     download_action_error: Option<String>,
     download_clear_confirmation: bool,
     download_security_confirmation: Option<PendingDownloadFileAction>,
@@ -182,6 +186,7 @@ impl ElyShell {
             workspace_picker_open: false,
             sidebar_hover_expanded: false,
             command_selected_index: 0,
+            sidebar_resize_origin: None,
             download_action_error: None,
             download_clear_confirmation: false,
             download_security_confirmation: None,