fix(grip): open hover popup without requiring mouse motion

Dwell timing only advanced when `ViewportMove` fired; a perfectly
still cursor never triggered the popup. Subscribe to animation
frames while `grip_hover` is set and dispatch `GripDwellTick` from
them so `update_grip_hover` re-checks the elapsed dwell each frame.
Subscription auto-stops as soon as the hover clears or the popup
opens.

src/app/mod.rs

@@ -453,6 +453,9 @@ pub enum Message {
     /// User picked an item in the multi-functional grip popup menu —
     /// the index is into `grip_popup.items`.
     GripMenuPick(usize),
+    /// Timer pulse while the cursor is dwelling on a grip; drives the
+    /// dwell-to-popup transition without requiring further mouse motion.
+    GripDwellTick,
     WindowResized(f32, f32),
     /// Enter pressed globally — finalises the active command (no text-input involvement).
     CommandFinalize,