feat(workspace): VS Code-style split panes, edge-to-edge session view

The console splits into up to four panes: a binary split tree of tmux
slots (client/src/lib/paneLayout.ts) drawn by PaneGrid.tsx, every pane an
absolutely placed sibling keyed by slot so splitting, swapping, resizing
or closing never remounts a live terminal. Ctrl/Cmd+click on a tab, a +
item or a sidebar session row opens it beside the focused pane (router
state, never the URL); a pane header drags onto another pane's edge to
split there or onto its middle to swap, picks another live session,
splits right/down (a not-shown session or a new terminal), or closes the
pane only. Dividers drag or take arrow keys. ?run= is the focused pane:
every layout change goes through change(), which pushes the URL with
focusOnly instead of an effect watching the focus (that ping-ponged with
open requests). The layout is kept per workspace in localStorage.

Only the focused pane's terminal takes the keyboard when it appears, and
the keyboard follows a focus change. A pane closes when its terminal
reports the exit; the run list prunes only a remembered layout on a
workspace's first read and a just-opened slot that never got listed, so
one empty read cannot wipe a split. A run ending on the home or a replay
closes its pane without leaving the page.

The session view now fills the page edge to edge: no page padding, the
tab bar and the right panel's activity bar one h-10 strip, and the tab
bar, pane headers and right panel on the sidebar/status bar colour
(surface-1) around terminals on the page colour.

client/src/components/lanes/LaneChangesPanel.tsx

@@ -984,7 +984,7 @@ export function LaneChangesPanel({
     <div data-testid="lane-changes-browse">
       {/* Pinned: the finder is what you reach for after scrolling a
                 long tree, not only from its top. */}
-      <div className="sticky top-0 z-10 flex items-center gap-1.5 bg-surface-0 pb-2">
+      <div className="sticky top-0 z-10 flex items-center gap-1.5 bg-surface-1 pb-2">
         <input
           value={query}
           onChange={(e) => setQuery(e.target.value)}
@@ -1320,7 +1320,7 @@ export function LaneChangesPanel({
               {(stagedCount > 0 || toStage.length > 0) && (
                 <div
                   data-testid="lane-changes-actions"
-                  className="sticky top-0 z-10 mb-1.5 flex items-center gap-1.5 border-b border-border-light bg-surface-0 pb-2"
+                  className="sticky top-0 z-10 mb-1.5 flex items-center gap-1.5 border-b border-border-light bg-surface-1 pb-2"
                 >
                   {/* ONE row: message, commit, then the two bulk index moves. */}
                   {stagedCount > 0 && (
@@ -1619,7 +1619,7 @@ export function LaneChangesPanel({
                           : t("changes.collapseFile", { path: f.path })
                       }
                       data-sticky-top=""
-                      className={`sticky top-0 z-10 flex cursor-pointer items-baseline gap-1.5 rounded-t bg-surface-1 px-2 py-1 hover:bg-surface-2 ${
+                      className={`sticky top-0 z-10 flex cursor-pointer items-baseline gap-1.5 rounded-t bg-surface-2 px-2 py-1 hover:bg-surface-3 ${
                         shut ? "" : "border-b border-border-light"
                       }`}
                     >