Phase 1: tabbed workspaces

Each tab is an independent tile tree; PTYs in non-active tabs keep
running (render-all-panes with visibility:hidden on inactive layers
so xterm.js's fit() still sees valid dimensions and the existing
per-pane resize dedupe absorbs no-op SIGWINCHes).

workspace.json shape goes from a bare TreeNode to
`{ version: 2, workspaces: [{ id, name, tree }] }` with a legacy v1
auto-wrap migration (the old single tree becomes one tab named
"Default").

App.tsx wraps the old single-tree state in workspace-aware state
but keeps `tree` / `setTree` / `activeLeafId` / `setActiveLeafId` as
identity-stable derived wrappers (reading currentWorkspaceId from a
ref), so the bulk of App.tsx stays unchanged.

XtermPane's initial term.focus() now checks `visibility !== "hidden"`
on the container so a pane mounting inside a hidden tab on app boot
doesn't yank focus away from the active tab. The focus poller is
scoped to the active workspace layer for the same reason.

Shortcuts: Ctrl+T new tab, Ctrl+Shift+T close current (window.confirm
when there are live panes), Ctrl+PageDown/PageUp navigate, Ctrl+1..9
switch to tab N. README + help overlay auto-generated from
shortcuts.ts.

79/79 vitest pass (7 new envelope-migration cases). tsc -b clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/shortcuts.ts

@@ -30,6 +30,21 @@ export const SHORTCUT_SECTIONS: ShortcutSection[] = [
       },
     ],
   },
+  {
+    title: "Tabs",
+    items: [
+      { keys: "Ctrl+T", description: "New tab (blank workspace, one pane)" },
+      {
+        keys: "Ctrl+Shift+T",
+        description: "Close current tab (confirms when the tab has live panes)",
+      },
+      {
+        keys: "Ctrl+PageDown / Ctrl+PageUp",
+        description: "Switch to next / previous tab",
+      },
+      { keys: "Ctrl+1 … Ctrl+9", description: "Switch to tab 1 … 9" },
+    ],
+  },
   {
     title: "Navigation",
     items: [
@@ -108,6 +123,10 @@ export const TIPS: TipSpec[] = [
     title: "Workspace persistence",
     body: "Layout, labels, distro choices, and SSH hosts auto-save to %APPDATA%/com.megaproxy.tiletopia (debounced 500ms). Closed panes don't come back — only the structure is restored, shells spawn fresh on next launch.",
   },
+  {
+    title: "Tabs (workspaces)",
+    body: "Each tab is an independent tile layout — useful for keeping one tab per project. PTYs in non-active tabs keep running (a Claude session in tab A keeps going while you work in tab B). New tab starts with one default-shell pane; close confirms when the tab has live panes. Tabs auto-save to the same workspace.json.",
+  },
   {
     title: "MCP server (let Claude drive the workspace)",
     body: "Titlebar 🤖 opens the MCP control panel. Start the server, then for Claude Desktop click 'Download .mcpb' and drag the file into Settings → Extensions — zero-config because the bundle reads your bearer token from %APPDATA% at launch (no copy-paste, survives token rotation). For Claude Code (terminal CLI) use the fallback snippet in the panel: it wires npx mcp-remote as a stdio shim because Claude Code's HTTP-MCP client ignores static bearer auth and tries OAuth instead. URL + token persist across restarts; Regenerate the token in the panel if it leaks. Default-deny per pane: toggle 🤖 on each pane's toolbar to expose it to MCP.",