feat(workspace): session view console as tabs, with New terminal

The 1/2/4 split view becomes a tab bar (ConsoleTabs.tsx): one tab per live
tmux slot (0-3) of the workspace, the active one being ?run=, every live tab's
pane kept mounted and hidden with CSS. The + menu opens a new tab in the first
free slot: New terminal (a plain shell - shell: true on POST
/api/lanes/:id/start, spawnShell, tool 'shell', no run_id/status change), any
installed enabled AI tool started bare, or Settings -> Agents. A tab's x stops
its session. splitViewStorage, &pane= and the sidebar's Ctrl/Cmd+click are gone.

server/routes/lanes.js

@@ -1785,6 +1785,31 @@ router.post("/:id/:action", sameOriginGuard, async (req, res) => {
             error: { code: "EBADSLOT", message: `slot must be an integer 0–${runs.MAX_PANE_SLOT}` },
           });
         }
+        // A Terminal tab: a plain shell in the slot. No agent is at work in
+        // it, so the lane's run_id/status stay as they are.
+        if (body.shell === true) {
+          const shellOutcome = await withLaneLock(lane.id, async () => {
+            const current = lanesLib.getLane(lane.id);
+            if (!current) return { missing: true };
+            const id = runs.runIdForLane(current.id, slot);
+            if (runs.getRun(id)?.status === "running") return { conflict: true };
+            startedRunId = runs.spawnShell({ laneId: current.id, slot, cwd: current.cwd }).id;
+            return {};
+          });
+          if (shellOutcome.missing) {
+            return res
+              .status(404)
+              .json({ error: { code: "ENOLANE", message: "workspace not found" } });
+          }
+          if (shellOutcome.conflict) {
+            return res
+              .status(409)
+              .json({
+                error: { code: "ERUNLIVE", message: "that tab already has a live session" },
+              });
+          }
+          break;
+        }
         // A session that ran in a worktree of its own resumes (or forks) THERE:
         // Claude finds a session by the folder it ran in. Resolved from the
         // session row, before the lock; anything else starts as it always did.