feat(gui): explain the disabled Connect button on hover

When the pasted code doesn't decode, Connect is greyed out; hovering it now
shows "Paste a valid share code first." so the disabled state is
self-explanatory, complementing the amber "doesn't look like a share code"
line under the field.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/gui/mod.rs

@@ -658,6 +658,7 @@ impl PixelPassApp {
                 decoded_id.is_some(),
                 egui::Button::new("Connect").min_size(egui::vec2(140.0, 36.0)),
             )
+            .on_disabled_hover_text("Paste a valid share code first.")
             .clicked();
         if decoded_id.is_some() && (connect_clicked || enter_pressed) {
             self.start_viewer(ui.ctx().clone());