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/api/router.go

@@ -49,6 +49,7 @@ func NewRouter(workspaces *service.WorkspaceService, files *service.FileService,
 	api.POST("/workspaces/:id/shell/restart", shellHandler.restart)
 	api.GET("/workspaces/:id/shell/status", shellHandler.status)
 	api.GET("/workspaces/:id/shell/ws", shellHandler.ws)
+	api.POST("/workspaces/:id/shell/resize", shellHandler.resize)
 	api.GET("/workspaces/:id/acp/status", acpHandler.status)
 	api.GET("/workspaces/:id/acp/history", acpHandler.history)
 	api.POST("/workspaces/:id/acp/prompt", acpHandler.prompt)

internal/api/shell_handler.go

@@ -53,6 +53,24 @@ func (h *shellHandler) restart(c *gin.Context) {
 	c.Status(http.StatusNoContent)
 }
 
+func (h *shellHandler) resize(c *gin.Context) {
+	id := c.Param("id")
+	var req model.ShellResizeRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		writeBadRequest(c, "invalid json")
+		return
+	}
+	if req.Cols <= 0 || req.Rows <= 0 {
+		writeBadRequest(c, "cols and rows must be positive")
+		return
+	}
+	if err := h.svc.Resize(id, req.Cols, req.Rows); err != nil {
+		writeError(c, err)
+		return
+	}
+	c.Status(http.StatusNoContent)
+}
+
 func (h *shellHandler) status(c *gin.Context) {
 	id := c.Param("id")
 	status, err := h.svc.Status(id)

internal/model/shell.go

@@ -6,3 +6,9 @@ type ShellStatusResponse struct {
 	Running     bool   `json:"running"`
 	PID         int    `json:"pid,omitempty"`
 }
+
+// ShellResizeRequest is the body for POST /shell/resize.
+type ShellResizeRequest struct {
+	Cols int `json:"cols"`
+	Rows int `json:"rows"`
+}

internal/service/shell_service.go

@@ -65,6 +65,19 @@ func (s *ShellService) Restart(workspaceID string) error {
 	return nil
 }
 
+// Resize resizes the PTY for the given workspace.
+func (s *ShellService) Resize(workspaceID string, cols, rows int) error {
+	if _, err := s.workspaces.Get(workspaceID); err != nil {
+		return err
+	}
+	if err := s.shells.Resize(workspaceID, cols, rows); err != nil {
+		s.logger.Error("shell resize failed", "workspace_id", workspaceID, "error", err)
+		return err
+	}
+	s.logger.Info("shell resized", "workspace_id", workspaceID, "cols", cols, "rows", rows)
+	return nil
+}
+
 // Status returns the shell status for the given workspace.
 func (s *ShellService) Status(workspaceID string) (shell.Status, error) {
 	if _, err := s.workspaces.Get(workspaceID); err != nil {

internal/shell/manager.go

@@ -29,6 +29,7 @@ type Manager interface {
 	Subscribe(workspaceID string) (Subscription, error)
 	Stdin(workspaceID string) (io.WriteCloser, error)
 	ExitStatus(workspaceID string) (ExitInfo, error)
+	Resize(workspaceID string, cols, rows int) error
 }
 
 // LocalManager implements Manager using local OS processes.
@@ -203,6 +204,25 @@ func (m *LocalManager) Restart(workspaceID string, workspaceRoot string) error {
 	return m.Start(workspaceID, workspaceRoot)
 }
 
+// Resize resizes the PTY for the given workspace.
+// Returns CodeNotFound if no session exists, or CodeBadRequest for invalid dimensions.
+func (m *LocalManager) Resize(workspaceID string, cols, rows int) error {
+	m.mu.Lock()
+	sess, ok := m.sessions[workspaceID]
+	m.mu.Unlock()
+	if !ok {
+		return util.New(util.CodeNotFound, "no running shell for workspace")
+	}
+	if cols <= 0 || rows <= 0 || cols > 10000 || rows > 10000 {
+		return util.New(util.CodeBadRequest, "invalid cols/rows")
+	}
+	f, ok := sess.Stdin.(*os.File)
+	if !ok {
+		return util.New(util.CodeInternal, "shell stdin is not a pty file")
+	}
+	return pty.Setsize(f, &pty.Winsize{Cols: uint16(cols), Rows: uint16(rows), X: 0, Y: 0})
+}
+
 // Status returns the current shell status for the workspace.
 func (m *LocalManager) Status(workspaceID string) Status {
 	m.mu.Lock()

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))
+	}
+}