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/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 {