feat(terminal): snapshot-replay on tab switches (xterm serialize + live pane capture)

Switching away from a session and back replayed only the server's byte
history. For TUI modes (codex especially) that shows just the latest
repaint — the idle banner — because the TUI drops earlier conversation
from its current frame. This restores the actual on-screen view.

Two complementary mechanisms:

- Client: load xterm's SerializeAddon and snapshot the rendered state
  (viewport + scrollback + colors) per session on switch-away, restoring
  it for an instant first paint on switch-back. The snapshot is only the
  first paint — the canonical /terminal frame is still fetched and
  reconciled (restoredSnapshot/clearedForBusy force the replay). Snapshots
  are LRU-bounded in memory (<=20) and persisted to localStorage
  (<=256KB each, <=10 sessions, stale-pruned) so they survive tab discard.

- Server: GET /api/sessions/:id/terminal prepends the live tmux pane
  buffer (via the existing captureActivePaneBuffer) ahead of the byte
  history, cleared between, so replay reflects the current frame.

Also fix formatPaneSnapshot dropping the rightmost column of every
captured row: it painted to cols - 1 out of caution about last-column
autowrap, but every row is followed by an absolute cursor-position CSI
that cancels xterm's pending-wrap, so painting the full width is safe.

The SerializeAddon is built from @xterm/addon-serialize (new dependency)
into the vendor bundle by postinstall.js (dev) and build.mjs (prod),
matching how the other xterm addons are vendored.

scripts/build.mjs

@@ -45,6 +45,7 @@ run('copy template', 'cp src/templates/case-template.md dist/templates/');
 run('xterm css', 'cp node_modules/@xterm/xterm/css/xterm.css dist/web/public/vendor/');
 run('xterm js', 'npx esbuild node_modules/@xterm/xterm/lib/xterm.js --minify --outfile=dist/web/public/vendor/xterm.min.js');
 run('xterm-addon-fit', 'npx esbuild node_modules/@xterm/addon-fit/lib/addon-fit.js --minify --outfile=dist/web/public/vendor/xterm-addon-fit.min.js');
+run('xterm-addon-serialize', 'npx esbuild node_modules/@xterm/addon-serialize/lib/addon-serialize.js --minify --outfile=dist/web/public/vendor/xterm-addon-serialize.min.js');
 run('xterm-addon-webgl', 'cp node_modules/@xterm/addon-webgl/lib/addon-webgl.js dist/web/public/vendor/xterm-addon-webgl.min.js');
 run('xterm-addon-unicode11', 'npx esbuild node_modules/@xterm/addon-unicode11/lib/addon-unicode11.js --minify --outfile=dist/web/public/vendor/xterm-addon-unicode11.min.js');
 run('xterm-zerolag-input', 'npx esbuild packages/xterm-zerolag-input/src/zerolag-input-addon.ts --bundle --minify --format=iife --global-name=XtermZerolagInput --outfile=dist/web/public/vendor/xterm-zerolag-input.js');

scripts/postinstall.js

@@ -252,6 +252,7 @@ if (isGlobalInstall) {
         const require = createRequire(import.meta.url);
         const xtermDir = join(require.resolve('@xterm/xterm'), '..', '..');
         const fitDir = join(require.resolve('@xterm/addon-fit'), '..', '..');
+        const serializeDir = join(require.resolve('@xterm/addon-serialize'), '..', '..');
         const webglDir = join(require.resolve('@xterm/addon-webgl'), '..', '..');
         const unicode11Dir = join(require.resolve('@xterm/addon-unicode11'), '..', '..');
         const vendorDir = join(srcDir, 'web', 'public', 'vendor');
@@ -264,12 +265,14 @@ if (isGlobalInstall) {
         try {
             execSync(`npx esbuild "${join(xtermDir, 'lib', 'xterm.js')}" --minify --outfile="${join(vendorDir, 'xterm.min.js')}"`, { stdio: 'pipe' });
             execSync(`npx esbuild "${join(fitDir, 'lib', 'addon-fit.js')}" --minify --outfile="${join(vendorDir, 'xterm-addon-fit.min.js')}"`, { stdio: 'pipe' });
+            execSync(`npx esbuild "${join(serializeDir, 'lib', 'addon-serialize.js')}" --minify --outfile="${join(vendorDir, 'xterm-addon-serialize.min.js')}"`, { stdio: 'pipe' });
             execSync(`npx esbuild "${join(unicode11Dir, 'lib', 'addon-unicode11.js')}" --minify --outfile="${join(vendorDir, 'xterm-addon-unicode11.min.js')}"`, { stdio: 'pipe' });
             console.log(colors.green('✓ xterm vendor files copied to src/web/public/vendor/'));
         } catch {
             // Fallback: copy unminified
             copyFileSync(join(xtermDir, 'lib', 'xterm.js'), join(vendorDir, 'xterm.min.js'));
             copyFileSync(join(fitDir, 'lib', 'addon-fit.js'), join(vendorDir, 'xterm-addon-fit.min.js'));
+            copyFileSync(join(serializeDir, 'lib', 'addon-serialize.js'), join(vendorDir, 'xterm-addon-serialize.min.js'));
             copyFileSync(join(unicode11Dir, 'lib', 'addon-unicode11.js'), join(vendorDir, 'xterm-addon-unicode11.min.js'));
             console.log(colors.green('✓ xterm vendor files copied') + colors.dim(' (unminified — esbuild not available)'));
         }