feat(shell): PTY resize via HTTP + frontend xterm.onResize hook

The PTY was hardcoded to 80x24 at start, so full-screen programs
(htop, vim, less, tmux, top) drew themselves for 80x24 regardless
of the actual xterm window. Add a path for the frontend to push the
real cols/rows to the backend, which calls pty.Setsize on the pty
file.

Backend:
- internal/shell/manager.go: Resize(workspaceID, cols, rows) added to
  the Manager interface and implemented on LocalManager. Looks up the
  session, type-asserts sess.Stdin.(*os.File), calls pty.Setsize.
  Validates cols/rows in [1, 10000] and returns CodeBadRequest on
  bad input, CodeNotFound when no session.
- internal/service/shell_service.go: ShellService.Resize wrapper that
  checks workspace existence first.
- internal/api/shell_handler.go: resize handler, 204 on success.
- internal/api/router.go: register POST /workspaces/:id/shell/resize.
- internal/model/shell.go: ShellResizeRequest{Cols, Rows}.
- internal/shell/manager_test.go: 3 new tests (valid resize via
  pty.Getsize round-trip, invalid size, missing session).

Frontend:
- web/src/lib/api/process.ts: useShellResize mutation hook.
- web/src/components/terminal/TerminalPanel.tsx: terminal.onResize
  subscription with 100ms debounce; filters 0x0; only fires when
  workspaceId is set; cleanup clears timeout + disposes listener.

E2E: 'stty size' after POST /shell/resize {cols:200, rows:50} returns
'50 200'. 0x40 → 400, missing workspace → 404, valid → 204.

Conversation: 019f360a-5eba-7c81-94d3-e5d58ad3c026

internal/shell/manager_test.go

@@ -9,6 +9,14 @@ import (
 	"time"
 )
 
+import (
+	"os"
+
+	"codespace/internal/util"
+
+	"github.com/creack/pty"
+)
+
 func startTestShell(t *testing.T) (*LocalManager, io.WriteCloser, Subscription) {
 	t.Helper()
 	mgr := NewManager("bash", []string{"-i"})
@@ -84,3 +92,45 @@ func TestShellPTYTermEnv(t *testing.T) {
 		t.Errorf("expected TERM=xterm-256color in output, got:\n%s", out)
 	}
 }
+
+func TestShellResizeUpdatesPTYSize(t *testing.T) {
+	mgr, stdin, sub := startTestShell(t)
+	_ = sub
+
+	f, ok := stdin.(*os.File)
+	if !ok {
+		t.Fatal("stdin is not a pty file")
+	}
+
+	if err := mgr.Resize("test-ws", 120, 40); err != nil {
+		t.Fatalf("Resize failed: %v", err)
+	}
+
+	rows, cols, err := pty.Getsize(f)
+	if err != nil {
+		t.Fatalf("Getsize failed: %v", err)
+	}
+	if cols != 120 || rows != 40 {
+		t.Errorf("expected cols=120 rows=40, got cols=%d rows=%d", cols, rows)
+	}
+}
+
+func TestShellResizeRejectsInvalidSize(t *testing.T) {
+	mgr, _, _ := startTestShell(t)
+
+	if err := mgr.Resize("test-ws", 0, 40); err == nil {
+		t.Fatal("expected error for invalid size")
+	} else if util.CodeOf(err) != util.CodeBadRequest {
+		t.Errorf("expected CodeBadRequest, got %v", util.CodeOf(err))
+	}
+}
+
+func TestShellResizeRejectsMissingSession(t *testing.T) {
+	mgr := NewManager("bash", []string{"-i"})
+
+	if err := mgr.Resize("missing-ws", 80, 24); err == nil {
+		t.Fatal("expected error for missing session")
+	} else if util.CodeOf(err) != util.CodeNotFound {
+		t.Errorf("expected CodeNotFound, got %v", util.CodeOf(err))
+	}
+}