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/run/LaneConsolePane.tsx

@@ -40,6 +40,8 @@ export interface LaneConsolePaneProps {
   onNewSession?: () => void;
   /** The pane's run exited (the tab bar opens another tab, or the home). */
   onRunExit?: () => void;
+  /** The terminal takes the keyboard when it appears (default). */
+  autoFocus?: boolean;
 }
 
 /** The tmux session a lane's tab owns (see server/lib/pty-run.js). */
@@ -57,6 +59,7 @@ export function LaneConsolePane({
   attachRequest,
   onNewSession,
   onRunExit,
+  autoFocus,
 }: LaneConsolePaneProps) {
   const { t } = useTranslation("run");
   const [handle, setHandle] = useState<RunHandle | null>(null);
@@ -169,6 +172,7 @@ export function LaneConsolePane({
           <TerminalView
             runId={handle.id}
             wsBaseUrl={window.location.origin.replace(/^http/, "ws")}
+            autoFocus={autoFocus}
             // The pane died (double Ctrl+C out of `claude`, `exit`, a crash) —
             // the tab bar moves on (another tab, or the home's prompt box)
             // rather than leaving a dead terminal the user has to click out of.