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/TerminalView.tsx

@@ -39,15 +39,20 @@ interface TerminalViewProps {
    *  There is nothing left to attach to, so the owner swaps this terminal back
    *  out for its run setup form instead of leaving a dead pane on screen. */
   onExit?: () => void;
+  /** Take the keyboard when it appears (default). Off for a split pane that
+   *  is not the focused one, or keys would land in whichever mounted last. */
+  autoFocus?: boolean;
 }
 
-export function TerminalView({ runId, wsBaseUrl, onExit }: TerminalViewProps) {
+export function TerminalView({ runId, wsBaseUrl, onExit, autoFocus = true }: TerminalViewProps) {
   const containerRef = useRef<HTMLDivElement>(null);
   // Read through a ref: an inline `onExit` from the parent changes identity on
   // every render, and as an effect dep it would tear the terminal down and
   // re-attach on each one.
   const onExitRef = useRef(onExit);
   onExitRef.current = onExit;
+  const autoFocusRef = useRef(autoFocus);
+  autoFocusRef.current = autoFocus;
   // Always the DARK theme's palette, even in light mode: Claude Code's TUI
   // assumes a dark background. Read through a ref at construction so a theme
   // change never re-runs the attach effect; the effect below recolours live.
@@ -143,7 +148,7 @@ export function TerminalView({ runId, wsBaseUrl, onExit }: TerminalViewProps) {
     // Take the keyboard as soon as the pane is on screen: the terminal IS the
     // page here, and having to click it first meant the first thing typed went
     // nowhere. A click elsewhere still moves focus away, as in any editor.
-    term.focus();
+    if (autoFocusRef.current) term.focus();
 
     // When the pane's program has NOT enabled wheel reporting, xterm falls back
     // to converting each wheel notch on an alt-screen buffer into a cursor-key