Add ability to restore previous layout.

src/style.css

@@ -79,6 +79,64 @@ button.playpen-header-button:hover,
   border-top: 1px solid @pp_border;
 }
 
+/* The restore-session offer, between the tab list and the footer. Rules of its
+   own rather than a reused row style: it is not a tab, and styling it as one
+   would put a fifth thing in a column of four that all select something.
+
+   Bordered top and bottom so it reads as a band across the sidebar rather than
+   a card floating in it — the footer below already draws the same hairline, and
+   the pair of them frame the offer without adding a third weight. The accent
+   tint is what makes it the one thing in the column asking to be clicked, kept
+   low enough that it does not compete with the selected row above it. */
+.playpen-restore {
+  padding: 6px 8px;
+  border-top: 1px solid @pp_border;
+  background-color: alpha(@pp_accent_strong, 0.08);
+}
+
+.playpen-restore-action {
+  padding: 5px 6px;
+  border-radius: 8px;
+  color: @pp_text_dim;
+}
+
+.playpen-restore-action:hover {
+  background-color: alpha(@pp_accent_strong, 0.16);
+  color: @pp_text;
+}
+
+/* The one accent-colored thing in the banner, so that what draws the eye is the
+   symbol for the action rather than the sentence describing it. */
+.playpen-restore-icon {
+  color: @pp_accent_strong;
+}
+
+.playpen-restore-title {
+  font-weight: bold;
+  font-size: 0.92em;
+}
+
+/* How many tabs are on offer, sized and dimmed like the sublabels in the
+   settings dialog: a caption on the action above it, not a second action. */
+.playpen-restore-detail {
+  color: @pp_text_faint;
+  font-size: 0.82em;
+}
+
+/* Quieter than the action beside it, and quieter still than a tab's close
+   button, which at least closes something you were looking at. Saying no to
+   this should be easy to find and hard to hit by accident. */
+.playpen-restore-dismiss {
+  min-width: 22px;
+  min-height: 22px;
+  padding: 0;
+  color: @pp_text_faint;
+}
+
+.playpen-restore-dismiss:hover {
+  color: @pp_text;
+}
+
 .playpen-settings-button {
   min-width: 26px;
   min-height: 26px;