feat(maestro): tighten mobile session UX

On phones (single tab, one pane at a time):
- drop the pane border/padding/focus-ring so the session fills the screen
- collapse the redundant tab bar; float a ☰ to keep the sidebar drawer reachable
- disable pull-to-refresh (overscroll-behavior) so terminal scroll wins

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/styles.css

@@ -183,6 +183,8 @@ html,
 body {
   height: 100%;
   margin: 0;
+  /* No pull-to-refresh / scroll chaining — terminal scroll wins on touch. */
+  overscroll-behavior: none;
 }
 
 body {
@@ -1914,6 +1916,11 @@ textarea {
   display: flex;
   flex-direction: column;
 }
+/* Mobile, single tab: the tab bar is pure chrome — collapse it (a floating ☰
+   inside the pane keeps the sidebar drawer reachable). */
+.mv-mobile-solo > .mv-tabs {
+  display: none;
+}
 
 /* tab bar */
 .mv-tabs {
@@ -2233,14 +2240,46 @@ textarea {
 
 .mv-mobile {
   position: absolute;
-  inset: var(--sp-2);
+  /* Flush to the edges — a single full-screen pane needs no canvas inset. */
+  inset: 0;
   display: flex;
   flex-direction: column;
-  gap: var(--sp-2);
+  gap: 0;
+}
+/* One pane fills the screen: no border, radius, or focus ring to box it in. */
+.mv-mobile > .mv-pane,
+.mv-mobile > .mv-pane.is-focused {
+  border: none;
+  border-radius: 0;
+  box-shadow: none;
+}
+/* Floating ☰ that replaces the collapsed tab bar, tucked into the header bar. */
+.mv-mobile-fab {
+  position: absolute;
+  top: 0;
+  left: 0;
+  z-index: var(--z-sticky);
+  display: inline-grid;
+  place-items: center;
+  width: 34px;
+  height: var(--bar-h);
+  border: none;
+  background: transparent;
+  color: var(--ink);
+  font-size: var(--fs-15);
+  cursor: pointer;
+}
+/* Make room for the ☰: shift the title (single pane) or pad the pill strip. */
+.mv-mobile-fab + .mv-pane .attach-title {
+  margin-left: 30px;
+}
+.mv-mobile-fab + .mv-panebar {
+  padding-left: 38px;
 }
 .mv-panebar {
   display: flex;
   gap: 4px;
+  padding: var(--sp-1) var(--sp-2);
   overflow-x: auto;
   scrollbar-width: none;
   flex: none;