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)